Energy Transfer and Confined Motion of Dyes Trapped in

ARTICLE pubs.acs.org/JPCC

Energy Transfer and Confined Motion of Dyes Trapped in Semiconducting Conjugated Polymer Nanoparticles Santanu Bhattacharyya, Bipattaran Paramanik, and Amitava Patra* Department of Materials Science, Indian Association for the Cultivation of Science, Kolkata 700 032, India ABSTRACT: This paper focuses on the spectroscopic studies on coumarin 153 (C153) dye encapsulated semiconducting [poly(N-vinylcarbazole) (PVK)] polymer nanoparticles using steady state and time-resolved spectroscopy. A significant blue shift (40 nm) and the enhancement of PL intensity of C153 dye are observed due to the encapsulation of C153 dye inside PVK nanoparticles. The quantum yield of encapsulated dye molecule increases with increasing the concentration of PVK, i.e., with size of the particle. The significant quenching (93%) of the PL spectrum and the shortening of the decay time of host PVK indicate the efficient energy transfer (82.5%) from PVK to C153 dye. The radiative decay rate of dye is found to increase with increasing the size of polymer nanoparticles indicating the increase of the refractive index inside polymer nanoparticle. The increase of lateral diffusion constant (DL) and the decrease of diffusion coefficient for wobbling motion (Dw) of dye molecules with an increase in the size of polymer nanoparticles suggest the increase in microviscosity and rigidity of the system.

’ INTRODUCTION The synthesis of highly efficient color tunable fluorescent nanoparticles has recently garnered considerable attention due to their potential use in high sensitive bioanalysis, rapid diagnostics, and effective therapeutics.1
4 Semiconducting quantum dots (QDs) should be excellent beacons for tagging with biomolecules, especially for multiplex optical detection and long-term imaging, because of several advantages, i.e., their narrower emission, broad absorbance, brighter luminescence, and better photostability.5
8 However, it has a problem for highly sensitive single biomolecular recognition due to power law blinking behavior of single QDs.9,10 Another problem is the exciton energy funnelling toward a trapped state or nonemissive states which decreases the emission intensity.11 Toxicity due to a heavy metal is another drawback to use for in vivo application.6 Aqueous dispersed dye doped conjugated polymer nanoparticles should be an efficient alternative in this respect.12
14 McNeill. et al. synthesized for the first time spectroscopic dye doped water-soluble conjugated polymer nanoparticles by a simple reprecipitation method. They have described the combined effects of energy diffusion and FRET in polymer nanoparticles doped with a variety of fluorescent dye molecules.15 Dye doped polymer nanoparticles have several advantages like superior fluorescent markers for single molecular studies, less toxicity for biocompatibility, and photostability.14,16
19 A solvatochromic and photochromic dye doped conjugated polymer nanoparticle is very important for the color tunable and photoswitchable properties.15,20
22 It is already demonstrated that dye doped silica nanoparticles are useful for biological applications.23,24 Application of organic semiconducting polymer based fluorescence energy transfer using a nanoscopic environment is still in r 2011 American Chemical Society

the embryonic stage, and further investigations in this field are necessary for in-depth understanding of the phenomenon for developing new challenging photonic devices.25
28 Warner demonstrated the energy transfer between MEH-PPV and PbS nanocrystals which follows the F€orster transfer process.29 Lutich and co-workers reported the F€orster energy transfer between CdTe nanocrystals and PDFD polymer molecules.30 Generally, the energy transfer (an exciton) or charge transfer (electrons and holes) from the host to the guest occurs by either Dexter or F€orster transfer processes. In the F€orster energy transfer process, the energy transfer mechanism is from a triplet state of the donor to the singlet state of the acceptor, and the rate depends on the overlap between donor emission and acceptor absorption. Fan et al.31 reported the hyperefficient energy transfer from conjugated polymers to gold nanoparticles. Kim et al.32 showed the enhancement of photoluminescence of the MEH-PPV/Au NP complex due to energy transfer or a local field effect. Kong and his co-workers also reported various optical properties of PVK polymer nanoparticles.33 The influence of the nature of semiconducting polymer hosts on the energy transfer from polymer nanoparticles to Au nanoparticles was also demonstrated very recently.34 In the present study, we have synthesized coumarin153 dye encapsulated PVK polymer nanoparticles with varying PVK concentration. Steady state and time-resolved fluorescence spectroscopies are used to investigate the energy transfer between host
guest and the local environments of coumarin 153 dye Received: May 4, 2011 Revised: September 16, 2011 Published: September 19, 2011 20832

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Figure 1. SEM images of C153 dye molecules doped 80 nm (a), 50 nm (b), and 10 nm (c) PVK polymer nanoparticles.

’ EXPERIMENTAL PROCEDURE PVK polymer nanoparticles were prepared by a typical reprecipitation method described elsewhere.33,34 Briefly, PVK was dissolved properly in dried THF to maintain the concentration of 0.5 mg/mL PVK. This THF solution (500 μL) was rapidly injected into 10 mL of distilled water under vigorous stirring for 5
10 min. Then, this solution was ultrasonicated for 30 min. As a result, an aqueous suspension of PVK polymer nanoparticles was obtained. To avoid aging of the polymer nanoparticles, the THF was evaporated from aqueous solution by partial vacuum evaporation for 1 h followed by filtration through a 0.2 μm filter paper. As a result, we obtained a stable aqueous suspension of PVK polymer nanoparticles which was stable for 5
7 days.

)

’ MATERIALS PVK [poly(9-vinylcarbazole)] (Aldrich), coumarin 153 (SigmaAldrich), distilled tetrahydrofuran (Merck), and deionized water (Merck) were used as received for our synthesis in the present study.

Next, we have prepared C153 dye doped PVK polymer nanoparticles by a similar reprecipitation method.15 Coumarin 153 dye (3  10
5 M) solution in THF was added to PVK solution and diluted to 2 mL by distilled THF such that the net concentration of PVK remains 0.5 mg/mL. At that time, the concentration of C153 in solution was 7.5  10
6 M. The THF solution was ultrasonicated for 30 min to obtain a clear mixing between C153 and PVK. Then, 500 μL of this THF solution (containing both PVK and C153) was rapidly injected into distilled water under vigorous stirring for 5
10 min. Similarly, ultrasonication, vacuum evaporation, and filtration through 0.2 μm filter paper was done. Thus, we obtained C153 dye doped PVK polymer nanoparticles in aqueous solution. To vary the dye concentration, we have varied the amount of C153 dye in stock THF solution by keeping the host PVK concentration fixed. To change the size of host PVK polymer nanoparticles, the concentration of PVK in stock THF solution varied from 0.5 mg/mL to 0.005 mg/mL. However, the concentration of C153 dye remains constant (7.5  10
6 M). Similar reprecipitation procedure was used to form the C153 dye doped different size PVK polymer nanoparticles. The morphological and size of the dye doped PVK nanoparticles were characterized by field emission scanning electron microscopy (FESEM, JEOL, JSM-6700F). Room temperature optical absorption spectra were taken by UV
vis spectrophotometer (Shimadzu). Room temperature photoluminescence spectra were taken by a Fluoromax-P (Horiba JOBIN YVON) photoluminescence spectrophotometer. In TCSPC measurement, we used picosecond NANO-LED IBH-340 for 340 nm excitation for PVK host and picosecond diode laser IBH-405 for 405 nm excitation of dye. For anisotropy decays, we used a motorized polarizer in the emission side. The analyzer was rotated by 90
at regular intervals. The parallel (I ) and perpendicular (I^) polarizations were collected alternatively until a certain peak difference between parallel (I ) and perpendicular decays were reached. The r(t) value was calculated by the following formula: )

molecules when encapsulated within the polymer nanoparticles. To the best of our knowledge, rotational relaxation studies, which are a convenient means of probing the microenvironment of the polymer nanoparticles, are not available in the literature. Timeresolved anisotropy study is essential to unravelling the origin of the rotational relaxation behavior of C153 dye inside the polymer nanosphere. The rotational motion of the dye molecule will be restricted due to encapsulation of dye in polymer nanoparticles. The anisotropy decay of the dye molecule in the polymer nanoparticles has been adequately described by reorientation times which are coupled to the wobbling motion and the lateral diffusion of the dye in the nanoparticles. Analysis of the timeresolved anisotropy decays will give better insight into the location and dynamics of the dye molecules.35 We are addressing a few important issues: How do the radiative decay, nonradiative decay, lateral diffusion constant, and diffusion coefficient for wobbling motion of the encapsulated dye molecules change with varying concentration of polymer host? Of particular interest to our research program is how the dynamics of the dye molecule encapsulated within polymer nanoparticles vary with changing microenvironments with the hope that such knowledge will enable us to construct efficient doped polymer nanomaterials for drug delivery and bioimaging applications.

rðtÞ ¼

I|| ðtÞ
GI^ ðtÞ I|| ðtÞ þ 2GI^ ðtÞ

ð1Þ

G factors were 0.68, 0.66, 0.69, and 0.66 for free C153 and C153 doped in 80 nm, 50, and 10 nm PVK polymer nanospheres, respectively. The analysis of the time-resolved data was done using IBH DAS, version 6, decay analysis software. The same software was used to analyze the anisotropy data. All experiments were done at room temperature. 20833

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Figure 2. (A) UV
vis spectra of pure PVK polymer nanoparticles (a), 0.3 wt % C153 doped PVK polymer nanoparticles (b). (B) Photoluminescence spectra of PVK polymer nanoparticles (a), C153 doped PVK polymer nanoparticles (b), and pure C153 in water (c).

dye doped) is above 90% after filtration. There is no significant change in UV
vis and PL spectra after filtrations. Again, we have dialyzed all the samples by using a dialysis membrane having molecular cutoff 1000 Da to understand whether the dye molecules are totally embedded in PVK polymer nanoparticles or are free in water. Then, we have taken the photoluminescence spectra of all the samples after and before dialysis, and we observed 3
6% change in intensity of the PL spectra, indicating that a negligible amount of free dye is released from the solution by dialysis. Hence, we can consider that the dye molecules are mostly confined in PVK polymer nanoparticles.

Figure 3. Photoluminescence spectra of C153 free in water (a) and dye encapsulated in 10 nm PVK (b), 50 nm PVK (c), and 80 nm PVK (d) polymer nanoparticles. Inset shows the photographs of C153 emission free in water (a) and encapsulated in 10 nm (b), 50 nm (c), and 80 nm (d) PVK polymer nanoparticles (excitation wavelength at 405 nm).

’ RESULTS AND DISCUSSION Figure 1 depicts the SEM images and size distributions of 0.3 wt % C153 dye doped PVK polymer nanoparticles with varying concentration of PVK. The average sizes of dye encapsulated PVK nanoparticles are 10, 50, and 80 nm for 0.005, 0.05, and 0.5 mg/mL PVK concentrations, respectively. The reason behind the nanoparticles formation of conjugated polymeric molecules has been discussed previously.34 First, PVK is dissolved in THF solvent, and then, this solution is injected rapidly to distilled water. As we know, THF quickly diffuses into water (as THF is water miscible) and PVK molecules immediately get close contact with poor solvent water. As a result, PVK molecules coiled up to form nanoparticles in water. Similarly, the rapid injection of the THF solution containing both hydrophobic C153 dye and polymer into distilled water can easily form dye doped polymer nanospheres. The dye to polymer ratio of precursor stock THF solution remains constant in aqueous solution of polymer nanoparticles (observed from UV
vis study). We have studied the loss of PVK due to filtration, and we have seen that the yield of polymer nanoparticles (pure and

’ STEADY STATE SPECTROSCOPIC STUDY Figure 2A represents the UV
vis absorption spectra of pure PVK polymer nanoparticles and 0.3 wt % C153 encapsulated PVK polymer nanoparticles. The absorption peak at 345 nm is due to the band gap of pure PVK (3.6
3.7 eV),36 and this band position does not shift even after doping the dye into polymer nanoparticles. The hump at 425 nm is due to coumarin 153 dye molecules. Figure 2B represents the PL spectra of pure coumarin 153 dye in water, dye encapsulated in PVK nanoparticles, and pure PVK nanoparticles in water at 340 nm wavelength of excitation. The PL peak at 537 nm is due to free C153 dye molecules, and the PL intensity is very low in water. It is to be noted that a significant blue shift (40 nm) of PL peak of C153 dye is observed in dye encapsulated in PVK nanoparticles. The blue shifting is due to change of polarity from polar to nonpolar environment. The intensity of the PL band of the dye significantly increases due to confinement of dye molecules in polymer nanoparticles. It is interesting to note that the PL band of PVK nanoparticles is significantly quenched (93%) in the presence of 0.3 wt % C153 dye molecules. The efficient superquenching of photoluminescence spectra of PVK indicates efficient energy transfer from host molecules to doped dye molecules. Figure 3 shows the normalized photoluminescence spectra of pure C153 in water and dye encapsulated in PVK nanoparticles of three different sizes where the samples are excited at 405 nm, the direct excitation of C153 dye. It is noted that a gradual blue shift of the PL peak of C153 dye is observed with increasing the concentration of PVK, i.e., with size of PVK nanoparticles. The quantum yield of C153 encapsulated polymer nanoparticles increases with 20834

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Figure 4. (A) Fluorescence intensity change of PVK host (a) and doped C153 (b) as a function of dye concentration. (All photoluminescence intensities are normalized to the 400 nm emission of host PVK). (B) Fluorescence quenching of PVK donor versus molar fraction of quenchers in doped polymer nanoparticles. The circles define the experimental results. The red solid line is plot fitted by Stern
Volmer equation.

acceptor molecules. Here, [A] is being expressed as molecular fraction of the quencher, and Ksv represents the number of the host molecule quenched by a single acceptor molecule. The Stern
Volmer plot (Figure 4B) suggests that about 11 polymer molecules are quenched by a single C153 dye molecule. McNeill et al. reported that 3
9 polymer molecules are quenched by a single dye molecule, depending upon the nature of dye.15 It is seen from Figure 5 that there is a good overlap between the absorption spectrum of C153 dye and emission spectrum of pure PVK nanoparticles, and we calculate the overlap integral using following equation: Z∞ Z∞ FD ðλÞεA ðλÞλ4 dλ= FD ðλÞ dλ ð3Þ JðλÞ ¼ 0

Figure 5. Overlap between the emission spectrum of PVK polymer nanoparticles (a) and absorption spectrum of C153 dye molecules (b).

increasing the concentration of PVK. The calculated quantum yields are 0.12, 0.17, 0.18, and 0.52 for C153 dye molecules in water, dye in 10, 50, and 80 nm PVK polymer nanoparticles, respectively (the concentrations of PVK in stock THF are 0.005, 0.05, and 0.5 mg/mL, respectively). The change of relative PL intensity of PVK and C153 with changing the encapsulated dye concentration has been given in Figure 4A (for PVK concentration 0.5 mg/mL in stock THF solution). All these data are taken with respect to the emission intensity of PVK at 400 nm. It is clearly seen that PL intensity of PVK nanoparticles decreases and the PL intensity of C153 dye increases with increasing the dye concentration. We have observed that the intensity ratio of PVK decreases up to 0.15 due to 0.4 wt % of C153 doping. On the other hand, the intensity ratio of C153 increases almost 2.5 due to 0.3 wt % of C153 doping. The intensity ratio for C153 does not change even after increasing the dye concentration. McNeill et al. calculated the number of the host molecule quenched by single acceptor molecule using the Stern
Volmer equation:15,37 F0 =F ¼ 1 þ Ksv ½A

ð2Þ

Here, F0 and F are fluorescence intensity due to PVK in the absence and presence of dye, respectively. Ksv is the Stern
Volmer quenching constant, and [A] is the concentration of the

0

The calculated overlap integral is 1.92  1015 M
1 cm
1 nm4, and the F€orster distance (R0) is calculated using the following equation: R0 ¼ 0:211½k2 η
4 ϕdonor JðλÞ1=6

ð4Þ

Here, k is the orientation factor, ϕdonor is quantum efficiency of the donor, and η is the refractive index of the medium.38 The calculated F€orster distance is 30.8 Å. 2

’ TIME-RESOLVED STUDY Figure 6A depicts the decay curves of pure PVK in the absence and in the presence of C153 dye (at 340 nm wavelength of excitation). In each case, the decay curves are fitted by multiexponential decay. The average decay time of pure PVK polymer nanoparticles is 1.11 ns. On the other hand, the average decay time of PVK (donor) decreases to 0.12 ns in the presence of 0.3 wt % C153 (Table 1). The calculated energy transfer efficiency is 82.5%. This significant amount of energy transfer occurs from host PVK to guest C153, and the rate of energy transfer is 74.32  108 s
1. To understand further the ET process, we have also analyzed the decay time of C153 dye after exciting at 340 nm (PVK excitation wavelength) in both free and doped systems. The decay time of pure C153 in water is 1.2 ns, and the decay time of C153 in PVK nanoparticles increases to 7.02 ns (Table 1). This significant increase in decay time of C153 dye is due to both resonance energy transfer and hydrophobic encapsulation in polymeric nanoparticles. Figure 7 20835

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Figure 6. (A) Decay curves of pure PVK polymer nanoparticles (a) and C153 dye doped PVK nanoparticles (b). (B) Decay curves of C153 dye in water (a) and dye in PVK polymer nanoparticles (b). (excitation wavelength is 340 nm).

Table 1. Fluorescence Decay Parameters of PVK and C153 Dye Encapsulated PVK Nanoparticlesa λem

τ1

τ2

τ3

(nm)

(ns) (a1)

(ns) (a2)

(ns) (a3)

Æτæ (ns)

PVK

400

3.24 (0.12)

16.36 (0.03)

0.24 (0.85)

1.11

PVK + C153 pure C153

400 537

0.10 (0.97) 1.2 (1)

0.73 (0.02)

6.17 (0.01)

0.12 1.2

PVK + C153

490

6.01 (0.89)

12.33 (0.11)

system

a

7.02

Excitation wavelength at 340 nm.

depicts the decay curves of pure C153 (excite at 405 nm) free in water and encapsulated in different sizes of PVK nanoparticles. This wavelength of excitation will excite only the dye molecules, not the host PVK. The decay time is 1.85 ns for free C153 dye. In the case of small PVK concentration (0.005 mg/mL), the decay curve is biexponential in nature. The fast component is 1.67 ns (76.2%), the slow component is 3.66 ns (23.8%), and the average decay time is 2.14 ns. In case of 0.05 mg/mL of PVK concentration, the fast component is 63.02% and the average decay time of C153 is 2.39 ns. However, the fast component is 44% in the case of 0.5 mg/mL PVK concentration. It reveals that the contribution of the fast component decreases with increasing the concentration of PVK, indicating the increasing hydrophobicity.39 Actually, the sizes of the polymer nanoparticles were tuned by only changing the concentration of PVK in nanoparticle precursor THF solution. With increasing concentration the rigidity and hydrophobicity of the polymer nanoparticle matrix increases. These affect the decay time of encapsulated dye molecules. The increase in decay time of the encapsulated dye molecules inside the PVK polymer nanoparticles is clearly understood from the modifications of both the radiative and nonradiative decay rate analysis. Webb et al.40 recently reported the enhancement of decay time of dye molecules in silica nanoparticles due to modifications of both radiative rate and nonradiative rate. It is seen from Table 2 that radiative rate increases from 6.48  107 s
1 to 12.7  107 s
1 for dye in water to dye in PVK (0.5 mg/mL) nanoparticles, respectively. This 2-fold enhancement of radiative decay rate of C153 may be due to increment of refractive index inside polymer nanoparticle. Toptygin et al.41 showed the effect of refractive index on the radiative decay rate of fluorophore. In the present

Figure 7. Decay curves of C153 dye in water (a) and encapsulated in 10 nm (b), 50 nm (c), and 80 nm (d) PVK polymer nanoparticles. (Excitation wavelength at 405 nm.)

system, the radiative rate increases because C153 dye molecules are encapsulated in PVK polymer matrix which is again surrounded by water. The refractive index of polymer matrix is comparatively higher than the dielectric medium, water. On the other hand, the nonradiative decay rate of C153 molecules decreases with increasing the PVK concentration. The nonradiative decay rates of C153 are 4.75  108 s
1 and 1.10  108 s
1 for dye in water and in PVK (0.5 mg/mL), respectively (Table 2). The change in nonradiative rate correlates with the restricted rotational mobility of the dye within the particles due to rigidity. Nonradiative rate corresponds to difference in rotational mobility of the dye within the particle.

’ TIME- RESOLVED ANISOTROPY STUDY Time-resolved anisotropy study is essential to unravelling the origin of the rotational relaxation behavior of C153 inside polymer nanoparticles. The rotation of dye molecule within the polymer nanoparticles will be restricted due to confinement. Therefore, analysis of the time-resolved anisotropy decays will give better insight into the location and dynamics of the dye molecules. Figure 8 depicts the anisotropic decay curves of C153 confined in different sizes of PVK polymer nanoparticles. 20836

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Table 2. Fluoresence Decay Parameters, Radiative and Nonradiative Decay Rate of C153 Dye and Dye Encapsulated PVK Nanoparticlesa system

a

λem (nm)

τ1 (ns) (a1)

τ2 (ns) (a2)

Æτæ (ns)

quantum yield of C153

kr  10
7 (s
1)

knr  10
8 (s
1)

C153 in water

537

1.85 (1.0)

1.85

0.12

6.48

4.75

C153 + PVK (0.005 mg/mL) C153 + PVK (0.05 mg/mL)

503 492

1.67 (0.762) 1.26 (0.63)

3.66 (0.24) 4.29 (0.37)

2.14 2.39

0.17 0.20

8.20 8.35

3.85 3.35

C153 + PVK (0.5 mg/mL)

490

1.67 (0.44)

6.01 (0.56)

4.09

0.52

12.71

1.10

Excitation wavelength at 405 nm.

Table 3. Anisotropic Decay Parameters of C153 Encapsulated PVK Nanoparticles τslow

τfast

Æτr æ

system

r0

(ns) (a)

(ns) (1
a)

(ns)

C153 + PVK (0.005 mg/mL)

0.28

1.52 (0.077)

0.12 (0.913)

0.23

C153 + PVK (0.05 mg/mL)

0.33

1.36 (0.34)

0.086 (0.66)

0.53

C153 + PVK (0.5 mg/mL)

0.37

1.44 (0.38)

0.095 (0.62)

0.62

following equation:42
44 1 1 1 ¼ þ τfast τE τslow 1 τslow Figure 8. Anisotropic decay curves of C153 dye encapsulated in 10 nm (a) and 80 nm (b) PVK polymer nanoparticles.

On the other hand, anisotropy decay of C153 molecules residing in PVK matrix increases with increasing the PVK concentration. Fluorescence anisotropy decay is fitted with a biexponential function:      t t rðtÞ ¼ r0 a exp
þ ð1
aÞexp
τslow τfast

ð5Þ

Here, τslow and τfast are the two reorientation times associated with the slow and fast motions of C153 molecules in PVK polymer nanoparticles. a is the pre-exponential factor which indicates the relative contributions of the slow and fast motions to the decay of the anisotropy. The average reorientation time Æτræ can be expressed as hτr i ¼ aτslow þ ð1
aÞτfast

1 1 þ τD τM

ð8Þ

τfast and τslow are the fast and slow reorientation times of C153 dye molecules. τM is the time constant for the overall rotation of polymer nanoparticles and can be estimated using the Stokes
Einstein
Debye relation with the stick boundary condition:38 τM ¼

4πηrh3 3kT

ð9Þ

Here, η is the viscosity of the medium. k and T are the Boltzmann constant and temperature in Kelvin. rh is the hydrodynamic radius of the polymer nanosphere and the values are 5, 25, and 40 nm. The τM values are 0.127, 15.9, and 65.15 μs for 10, 50, and 80 nm PVK nanoparticle encapsulated systems (Table 4). As all the τM values are in the microsecond region, it has a negligible effect on lateral diffusion phenomena. As mentioned earlier, τslow represents the time constant for lateral diffusion and is related to the lateral diffusion constant by the relation DL ¼

ð6Þ

The average reorientation times are 0.23, 0.52, and 0.62 ns for dye encapsulated in 10, 50, and 80 nm sizes of PVK nanoparticles, respectively. All these data are shown in Table 3. We also calculated several rotational parameters using a two step model, i.e., the slow lateral diffusion (τD) of C153 molecules on the surface of the polymer nanoparticles and the fast wobbling motion of C153 inside polymer nanosphere which are coupled with the overall rotation of the polymer nanosphere in aqueous suspensions. The wobbling in a cone model demonstrates the internal motion of the C153 molecules (τE) in terms of cone angle (θ0) and the wobbling diffusion coefficient (Dw). Assuming that the slow and the fast motions are separable we have measured the above rotational parameters by applying the

¼

ð7Þ

rh2 6τD

ð10Þ

The lateral diffusion constants are 0.27  10
4, 7.66  10
4, and 18.52  10
4 cm2/s for 10, 50, and 80 nm PVK polymer nanoparticle confined systems, respectively. Order parameter (S) has a magnitude that considered as a measure of the spatial restriction of the probe dye molecules, and its value varies from 0 to 1 depending upon unrestricted and restricted rotation of dye molecules, respectively. The order parameter is related with the amplitude of the slower component of anisotropy decay by the following equation:45 S2 ¼ a

ð11Þ

The order values are 0.28, 0.58, and 0.62 for 0.005, 0.05, and 0.5 mg/mL of PVK concentration, respectively. Higher value for 20837

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Table 4. Different Rotational Parameters for C153 Dye Encapsulated PVK Nanoparticles system

τM (μs)

τE (ns)

τD (ns)

Dw  10
9 (s
1)

DL  104 (cm2/s)

θ0 (deg)

S

C153 + PVK (10 nm)

0.127

0.1303

1.53

1.83

0.27

66.4

0.28

C153 + PVK (50 nm)

15.9

0.10

1.36

1.48

7.66

47.1

0.58

C153 + PVK (80 nm)

65.15

0.102

1.44

1.37

18.52

43.9

0.62

the order parameters indicates the dye molecules are experiencing restricted rotation in nanoparticles. Therefore, the mobility of the encapsulated dye molecules decreases with increasing the size of particles. According to the “wobbling in a cone model” the C153 molecules wobble inside a cone of semiangle θ. The relation between order parameter and θ is as follows: S ¼ 0:5 cos θð1 þ cos θÞ

ð12Þ

The calculated θ values are 66.4
, 47.1
, and 43.9
in the cases of 10, 50, and 80 nm polymer nanospheres, respectively. The diffusion coefficient for wobbling motion Dw for three different systems is calculated by using the following equation:46 Dw ¼

"     1 cos2 θð1 þ cos θÞ2 1 þ cos θ ð1
cos θ þ ln 2 ½ð1
S ÞτE  2 2 2ðcos θ
1Þ

ð1
cos θÞ þ ð6 þ 8 cos θ
cos2 θ
12 cos3 θ
7 cos4 θÞ 24



ð13Þ It is seen from Table 4 that the diffusion coefficient for wobbling motion Dw decreases with increasing the concentration of PVK, i.e., the size of the polymer nanoparticles, indicating the microviscosity experienced by the dye molecules, increases. According to wobbling in a cone model the dye molecules wobble freely inside a cone of semiangle θ, which depends on the order parameter/ restriction parameter of the medium. As we increase the size (as well as concentration) of PVK polymer nanoparticles, the restriction parameter increases. Thus, the probability for the free wobble of dye molecules decreases.42
44 The value of the cone angle for wobbling motion also decreases with increasing the size of the polymer nanoparticles. As a result, the diffusion coefficient for the wobbling motion decreases with increase in the size of PVK polymer nanoparticles.

’ CONCLUSIONS In our paper, we have synthesized coumarin 153 dye encapsulated PVK polymer nanoparticles and have demonstrated a significant blue shift (40 nm) and the enhancement of PL intensity of C153 dye due to the encapsulation of C153 dye inside PVK nanoparticles. The superquenching (93%) of PL spectrum and shortening of decay time of host PVK indicate the efficient energy transfer (82.5%) from PVK to C153 dye. It is found out that the restriction factor and the radiative decay rate increase and the noradiative decay rate and wobbling motion of dye decrease with increasing the PVK concentration. The anisotropy decay of the dye molecule in the polymer nanoparticles has been adequately described by a sum of two exponentials with fast and slow reorientation times. The fast and slow components have been ascribed to the wobbling motion and the lateral diffusion of the dye in the nanoparticles, respectively, and both of these motions are coupled to the rotation of the nanoparticle as a whole. Analysis suggests that microviscosity and rigidity increase with increasing the PVK concentration which

will have an influence on restricted motion of dye molecules in polymer nanoparticles and as a result the enhancement of anisotropy decay.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Phone: (91)-33-2473-4971. Fax: (91)-33-2473-280.

’ ACKNOWLEDGMENT The CSIR and “Ramanujan Fellowship” are gratefully acknowledged for financial support. S.B. and B.P. thank CSIR for awarding fellowship. ’ REFERENCES (1) Roming, M.; L€unsdorf, H.; Dittmar, K. E. J.; Feldmann, C. Angew Chem., Int. Ed. 2010, 49, 632–637. (2) Chatterjee, D. K.; Gnanasammandhan, M. K.; Zhang, Y. Small 2010, 24, 2781–2795. (3) Ha, S. W.; Camalier, C. E.; Beck, G. R., Jr.; Lee, J. K. Chem. Commun. 2009, 2881–2883. (4) Yong, K. T.; Roy, I.; Hu, R.; Ding., H.; Cai, H. X.; Zhu, J.; Zhang, X. H.; Bergey, E. J.; Prasad, P. N. Integr. Biol. 2010, 2, 121–129. (5) Chen, W. C. W.; Nie, S. Science 1998, 281, 2016–2018. (6) Michalet, X.; Pinaud, F. F.; Bentolila, L. A.; Tsay, J. M.; Doose, S.; Li, J. J.; Sundaresan, G.; Wu, A. M.; Gambhir, S. S.; Weiss, S. Science 2005, 307, 538–544. (7) Yezhelyev, M. V.; Al-Hajj, A.; Morris, C.; Marcus, A. I.; Liu, T.; Lewis, M.; Cohen, C.; Zrazhevskiy, P.; Simons, J. W.; Rogatko, A.; Nie, S.; Gao, X.; O'Regan, R. M. Adv. Mater. 2007, 19, 3146–3151. (8) Bouzigues, C.; Morel, M.; Triller, A.; Dahan, M. Proc. Natl. Acad. Sci. U.S.A 2007, 104, 11251–11256. (9) Durisic, N.; Bachir, A. I.; Kolin, D. L.; Hebert, B.; Lagerholm, B. C.; Grutter, P.; Wiseman, P. W. Biophys. J. 2007, 93, 1338–1346. (10) Jaiswal, J. K.; Simon, S. M. Trends Cell Biol. 2004, 14, 497–504. (11) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chem. Rev. 2005, 105, 1025–1102. (12) Mailander, V.; Landfester, K. Biomacromolecules 2009, 10, 2379–2400. (13) Jin, Y.; Ye, F.; Zeigler, M.; Wu, C.; Chiu, D. T. ACS Nano 2011, 5, 1468–1475. (14) Wu, W. C.; Chen, C. Y.; Tian, Y.; Jang, S. H.; Hong, Y.; Liu, Y.; Hu, R.; Tang, B. Z.; Lee, Y. T.; Chen, C. T.; Chen, W. C.; Jen, A. K. Y. Adv. Funct. Mater. 2010, 20, 1413–1423. (15) Wu, C.; Zheng, Y.; Szymanski, C.; McNeill, J. J. Phys. Chem. C 2008, 112, 1772–1781. (16) Wu, C.; Bull, B.; Christensen, K.; McNeill, J. Angew. Chem., Int. Ed. 2009, 48, 2741–2745. (17) Kumar, R.; Ohulchanskyy, T. Y.; Roy, I.; Gupta, S. K.; Borek, C.; Thompson, M. E.; Prasad, P. N. ACS Appl. Mater. Interfaces 2009, 1, 1474–1481. (18) Tuncel, D.; Demir, H. V. Nanoscacle 2010, 2, 484–494. (19) Grimland, J. L.; Wu, C.; Ramoutar, R. R.; Brumaghim, J. L.; McNeill, J. Nanoscale 2011, 3, 1451–1455. 20838

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