PbSe Nanostructures

Feb 23, 2015 - Photophysical Aspects of Varying Zn2+/ PbSe Nanostructures Mediated by RNA Leading to the Formation of Honeycomb-like Novel Porous Morp...
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Photophysical Aspects of Varying Zn2+/ PbSe Nanostructures Mediated by RNA Leading to the Formation of Honeycomb-like Novel Porous Morphology Anil Kumar,* Bhupender Singh, and Komal Gupta Department of Chemistry Indian Institute of Technology Roorkee, Roorkee - 247667, India S Supporting Information *

ABSTRACT: This paper reports the effect of the addition of varied concentrations of Zn2+ on the photophysical properties of RNA-mediated PbSe nanostructures. An increasing addition of Zn2+ results in diminishing of the excitonic feature in the optical spectrum associated with a decrease in red emission with a simultaneous increase in the near-infrared (NIR) region up to 10 × 10−5 mol dm−3. It causes the emission lifetime to decrease from 255 to 208 ns at 770 nm and increase from 10.4 to 17.6 ns at 1000 nm. The addition of Zn2+ changes the nature of Q-PbSe from direct to indirect band gap semiconductor by creating different surface states within its band gap, inducing additional transitions. It is understood to facilitate the phonon-assisted relaxation populating the deeper traps responsible for enhanced NIR fluorescence. The adsorption capacity of aged Zn2+/PbSe is enhanced for Nile blue (NB) as compared to its fresh samples due to increased porosity. The excitation of PbSe with energy greater than the bandgap energy in NB-supported PbSe nanostructures results in an energy transfer from excited PbSe to NB involving multiple exciton generation per photon. The porosity, enhanced adsorption capacity with fairly high emission yield, and lifetime in the NIR region give Zn2+/PbSe significant potential as a synthesized nanosystem for tissue and bioimaging applications. it generated the fluorescing honeycomb-like structure in the process of self-assembly. The present work reports the optical and photophysical properties of PbSe nanostructures as a function of the concentrations of Zn2+, focusing specifically on the conditions leading to the formation of honeycomb like morphology. These investigations have been performed by monitoring the absorption and fluorescence behavior in the visible and nearinfrared (NIR) ranges. It also explores the application of these nanostructures for supporting the water-soluble dye, Nile blue (NB). NB gets adsorbed on their surface and quenches the red and NIR emission along with the simultaneous development of a new peak arising due to the excitation of the dye on its surface. A correlation between morphology and photophysics has also been analyzed. Such PbSe nanostructures may find potential for their usage in the areas of extended sensitization, optoelectronics, and biomedical and fluorescence imaging.1−10

1. INTRODUCTION In recent years, IV−VI semiconductors have been investigated intensively because of their tremendous potential in optoelectronic,1−5 IR technology,6 solar cell,7,8 biological labeling, fluorescence imaging,9,10 and detection. There have been several reports on the synthesis and optical properties on PbS, PbSe, and PbTe nanostructures.5,11 Among these, PbSe having the smallest band gap (0.278 eV), relatively high excitonic Bohr radius (46 nm), and small effective masses of the carriers [e− (0.047) and h+ (0.040)]12 may lead to a strong quantum confinement effect,13−16 thus making it interesting for controlling optical properties and photophysics. In recent years, capping of nanomaterials with biomolecules have been explored extensively for enhancing their optical, photophysical, and surface properties.17 In earlier work, the synthesis and optical properties of RNAmediated PbSe in the varied environment of pH, molar ratio of Pb/Se, and the presence of different counterions like Pb2+15 and Mg2+16 have been reported. Lately, an interesting investigation of Zn2+ ions as counterions on the production of PbSe nanostructures has appeared18 in which at lower concentrations of Zn2+ (≤2.5 × 10−5 mol dm−3), it produced spherical nanoparticles, whereas, at higher concentrations (>5.0 × 10−5 to 10.0 × 10−5 mol dm−3) some pits and holes are generated. Under specific conditions of pH (≤8.5), molar ratios of Pb/Se (2:1) and concentrations of RNA (0.022 g/100 mL), © 2015 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Reagents. Ribonucleic acid derived from Torula yeast type VI (RNA), NB, and Se powder (99.99%, 100 mesh) Received: December 5, 2014 Revised: February 21, 2015 Published: February 23, 2015 6314

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Figure 1. Effect of addition of Zn2+ on optical absorbance and fluorescence spectra of RNA-mediated PbSe in the visible region (a) and in the NIR region (b).

(UV−vis−NIR spectrophotometers/spectrofluorophotometers, IR, NMR, and circular dichroism (CD)), microscopic tools (atomic force microscopy (AFM), field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM)), and X-ray photoelectron spectroscopy (XPS). ImageJ software was used for image analysis. It was specifically used for 3D viewing of porosity in the nano/ microstructures recorded by FESEM and TEM. NOVA software was used for analyzing the pore/grain size of the AFM images. The morphological changes for this sample was analyzed as a function of the concentration of Zn2+ using AFM spectroscopy, which resulted in a change in morphology from spherical nanoparticles up to 99.99%) (Sigma, India), lead acetate (Qualigens), sodium hydroxide, and sodium borohydride (Merck), zinc acetate (Fluka), styryl 13 (Exciton), and all other chemicals were of analytical grade and used as received. The used RNA sample was a heterogeneous mixture of RNA molecules of varied molecular weight(s) and length(s), and no specific sequence was employed. 2.2. Equipment. Optical absorption spectra were recorded on Shimadzu UV2100S and Carry 5000 spectrophotometers in 1 cm quartz cell. Steady state fluorescence and anisotropic measurements were made on a Shimadzu RF-5301PC, Horiba Jobin Yvon Nanolog and Edinburgh FLS-980 spectrofluorophotometers equipped with a 450 W xenon lamp source and FL-1044 polarizer L-format accessories having fully automated dual polarizer. Fluorescence lifetimes, time-resolved emission spectra (TRES), and time-resolved fluorescence anisotropic measurements in nanosecond and microsecond time domains were recorded in 1 cm quartz cell on a Horiba Jobin Yvon “FluoroCube Fluorescence Lifetime System” equipped with NanoLEDs and LDs as excitation source(s) and an automated polarization accessory (Model 5000 U-02). Lifetimes in UV− visible (300−800 nm) and NIR ranges were recorded by employing thermoelectrically cooled TBX-04-D and Hamamatsu H10330-75 (No. BB0062) detectors, respectively. The later detector was operated at −793 V to detect the emitted photons in the NIR range (950−1700 nm). 2.3. Synthesis of RNA Capped Zn2+/ PbSe Nanostructures. RNA capped Zn2+/PbSe nanostructures were synthesized using a previously reported method under inert environment at 4 °C by maintaining a pH of 8.5 at each step.18 A varied amount of Zn2+ solution [(1.0−15.0) × 10−5 mol dm−3] was added to the deaerated solution of RNA (0.022g/100 mL) prior to the addition of Pb2+ (150 μL of 0.1 mol dm−3). It was followed by the addition of freshly prepared NaHSe from the sides of the vessel under vigorous stirring, turning the solution reddish brown instantaneously. The resulting solution was purged with nitrogen gas strongly for about 10 min followed by the dropwise addition of 150 μL of 0.1 mol dm−3 excess Pb2+ solution in order to make up the effective concentrations of PbSe and Pb2+ at 1.5 × 10−4 mol dm−3 each, and Pb/Se = 2. 2.4. Methodology. The characterization of these nanostructures was carried out by employing various spectroscopic 6315

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Figure 2. 3D excitation−fluorescence spectra of ZnSP1 in the visible (a) and NIR (b) regions.

3. CHARACTERIZATION OF ZN2+ INDUCED RNA-MEDIATED ZN2+/ PBSE NANOSTRUCTURES 3.1. Optical Absorption and Fluorescence. The optical absorption spectra of fresh colloidal PbSe containing different amounts of Zn2+ (1.0 × 10−5 to 15 × 10−5 mol dm−3) in the UV-visible and NIR regions (200−1050 nm) are shown in Figure 1a,b. The amount of Zn2+ was optimized by monitoring the optical absorption and fluorescence in visible and NIR regions (Figure 1a,b). These spectra show that an increase in the amount of Zn2+ results in a complex change in the absorption behavior up to about 740 nm (1.68 eV). With increasing Zn2+, the weak excitonic bands at 420 nm (2.95 eV) and 670 nm (1.85 eV), observed with RNA-mediated PbSe alone16 (in the absence of Zn2+), become increasingly less prominent along with a simultaneous increase in the absorption beyond 740 nm (>1.57 eV) up to 10 × 10−5 mol dm−3 (Figure 1a). It causes an increase in the onset of absorption from 1025 (1.21 eV) to 1050 nm (1.18 eV) in the NIR range (Figure 1b). An increase in the concentration beyond >10 × 10−5 mol dm−3, however, brings a decrease in the absorption in the entire UV− vis−NIR range. The excitation wavelength for these nanostructures was worked out by recording the 3D fluorescence in visible (680−900 nm) (Figure 2a) and NIR (850−1200 nm) (Figure 2b) regions as a function of varied wavelengths from 350−670 nm. From these experiments, the excitation wavelength corresponding to the maximum quantum yield of fluorescence was found to be 670 nm. The fluorescence behavior of these nanostructures was also recorded as a function of [Zn2+] (Figure 1a,b). An increase in the concentration of Zn2+ from 1 × 10−5 to 10 × 10−5 mol dm−3 brings a regular decrease in the fluorescence intensity in the red region along with a simultaneous increase in the fluorescence intensity in the NIR range, but any further increase in its concentration results in a decrease in the intensity in both of the regions. The sample containing 10 × 10−5 mol dm−3 of Zn2+ was found to correspond to the maximum quantum efficiency in both the visible (Φem = 0.01) and NIR (Φem = 0.5) ranges. This

sample has been abbreviated as ZnSP1. Aging of this sample exhibits a change in the absorption behavior, which is in the same direction as was observed upon increasing [Zn2+] (Figure 3). It suggests that during the process of aging, interaction of

Figure 3. Absorption and fluorescence spectra of ZnSP1: fresh (a) and aged (b). Reprinted with permission from ref 18. Copyright 2013 American Chemical Society.

PbSe with Zn2+ species becomes more prominent. The fluorescence due to aged sample, however, depicted a blue shift in the fluorescence bands in red and NIR ranges from 806 nm (1.54 eV) to 769 nm (1.61 eV), and 944 nm (1.31 eV) and 1050 nm (1.18 eV) bands to 905 nm (1.37 eV) and 1006 nm (1.23 eV), respectively, with a significant reduction in the fluorescence intensity (Figure 3). 3.2. Fluorescence Lifetime Measurements. Steady state changes in the fluorescence were further analyzed by monitoring the dynamics of charge carriers by following the time-resolved fluorescence decay both in the visible (Figure 4A; Table 1) and NIR (Figure 4B; Table 2) regions as a function of [Zn2+] (1.0 × 10−5 to 15.0 × 10−5 mol dm−3). In the visible 6316

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Figure 4. Effect of addition of [Zn2+] on the fluorescence lifetime decay curve of RNA-templated PbSe at (A) λex = 635 nm; λfl = 770 nm and (B) λex = 635 nm; λfl = 1000 nm. (C) Fluorescence lifetime decay curves of ZnSP: fresh (a) and aged (b) (λex = 635 nm; λfl = 770 nm).

Table 1. Effect of the Addition of Zn2+ on Fluorescence Lifetime (ns) of RNA-Templated PbSe in the Visible Region (λex= 635 nm; λfl = 770 nm) lifetime (ns) [Zn2+] (× 10−5 mol dm−3) 1.0 2.5 5.0 10.0 (ZnSP1 Fresh) 10.0 (ZnSP1 aged) 15.0

τ1 2.39 1.85 0.96 0.44 0.18 0.39

(0.8) (1.0) (2.53) (12.9) (225.5) (21.2)

τ2

emission % 2.27 2.48 3.66 16.3 57.3 34.1

66.1 65.0 58.3 56.8 29.0 62.8

emission %

(0.34) (0.31) (0.31) (0.14) (0.02) (0.07)

26.5 26.73 26.77 22.8 6.17 19.51

τ3 333.5 334.1 303.8 320.6 261.8 348.1

(0.18) (0.16) (0.15) (0.06) (0.01) (0.03)

emission %

⟨τ⟩ (ns)

χ2

71.2 70.8 69.1 60.9 36.5 46.4

255 254 226 208 97.4 174

1.3 1.3 1.2 1.2 1.2 1.1

Values in brackets are pre-exponential factors corresponding to respective τ.

Table 2. Effect of Addition of Zn2+ on Fluorescence Lifetime (ns) of RNA-Templated PbSe in the NIR Region (λex = 635 nm; λfl = 1000 nm) [Zn2+] ( × 10−5 mol dm−3)

emission %

⟨τ⟩ (ns)

χ2

1.0 2.5 5.0 10.0 (ZnSP1 fresh) 10.0 (ZnSP1 aged) 15.0

100 100 100 100 100 100

10.4 13.2 16.4 17.6 15.1 9.4

1.07 1.03 1.08 1.04 1.06 0.97

and hundreds of nanoseconds time domains. An examination of these data clearly shows a gradual increase and decrease in the % emission for different components. For the first component, the percentage emission is increased from 2.27 to 34.11 for a change in increase in concentration of Zn2+ from 1.0 × 10−5 mol dm−3 to 15 × 10−5 mol dm−3, whereas the % emission due to the third component decreased from 71.2 to 46.4. An increase in the % emission of the first component, and a decrease for the second and third components suggests that the addition of Zn2+ creates both shallow as well as deeper traps (Table 1). The observation of a gradual decrease in average lifetime ⟨τ⟩ from 255 to 174 ns with increasing Zn 2+ concentration from 1.0 × 10−5 to 15.0 × 10−5 mol dm−3, indicates that electrons and holes do not stay for a long time in

region, all the decay curves followed three exponential kinetics having time constants in the nanoseconds, tens of nanoseconds, 6317

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Figure 5. TRES spectra along with 3D images of fresh ZnSP1 (a and a′) and aged ZnSP1 (b and b′).

delays worked out from the fluorescent decay curves under the used experimental conditions (Figure 5a,a′). These decay curves clearly show the presence of three intermediates. The first intermediate depicts high fluorescence intensity around 650 nm, and relatively lesser intensity in the red region. After about 170 ns, the fluorescence decays at lower wavelength(s) to produce a broad fluorescence band at longer wavelengths having maxima around 730 nm. After about 280 ns delay, the fluorescence at higher energy almost vanishes, and it forms a red-shifted fluorescence band having maxima at about 736 nm. Thereafter, it was found to simply decay without any further change in the spectral behavior. These data suggest the presence of three intermediates contributing to the fluorescence. The aged sample also depicts very similar spectral changes in the fluorescence decay process (Figure 5b,b′) as was observed for the fresh ZnSP1 (Figure 5a,a′) except that the intensity of different fluorescence spectra at different delays depicted relatively lesser fluorescent intensity. The fluorescent transients recorded after 115 ns delay shows the high fluorescence intensity at lower wavelength and a very broad fluorescence in the red region. After 175 ns delay, the fluorescence intensity at lower wavelength decays to produce a relatively more prominent band in the red region having fluorescence maxima at 735 nm. This spectrum is further changed over a period of about 300 ns to produce another spectrum, which depicted much less fluorescence intensity at

the shallow traps and relax rapidly to the deeper nonradiative traps to cause a decrease in the fluorescence lifetime. Upon aging of ZnSP1, ⟨τ⟩ is decreased from 208 to 97.4 ns, respectively (Figure 4C; Table 1). In the NIR region, the fluorescence decay was monitored kinetically at 1000 nm (1.24 eV). In these curves, the latter part of this decay could only be differentiated with the electronic signal obtained and was found to follow the first exponential kinetics. An increase in [Zn2+] resulted in a regular increase in ⟨τ⟩ with increasing [Zn2+] up to 10.0 × 10−5 mol dm−3, and, thereafter, it starts decreasing (Table 2). For the optimized sample ZnSP1, the fluorescence lifetime was estimated to be 17.6 ns and reduces to 15.1 ns upon aging for 25 days. From the lifetime data, the depth of traps corresponding to different lifetime components were estimated to vary from 209 to 380 meV (Tables S1A and S1B). From both the steady state and time-resolved emission studies, the best fluorescence features are observed for the PbSe sample containing 10.0 × 10−5 mol dm−3 of Zn2+ (ZnSP1). It is exactly this concentration under which the honeycomb-like structure has been reported and has also been verified in the present work (Figure S1). Therefore, further studies have been performed only with this sample. 3.3. Time Resolved Fluorescence Studies. The dynamics of the charge carriers for ZnSP1 was further analyzed by recording the time-resolved fluorescence spectra at different 6318

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The Journal of Physical Chemistry C lower wavelengths and fluorescence maxima is now blue-shifted to 725 nm. From the examination of these spectral changes, it is quite apparent that aging of these particles produce intermediates having much lower florescence intensity along with a blue-shifted fluorescence band at longer delays. These changes seems to corroborate with the steady state fluorescence behavior, where, for the fresh particles, red-shifted less intense fluorescence bands are produced with increasing Zn2+, and, upon aging, the intensity of fluorescence decreases associated with a blue shift in the fluorescence maxima. 3.4. Fluorescence Anisotropy Measurement. From the steady state measurement for the fresh ZnSP1, the anisotropy value was obtained to be 0.06 (Table 3), which changes Table 3. Steady State Anisotropy and Polarization of Fresh and Aged ZnSP1 sample

anisotropy

polarization

ZnSP1 fresh ZnSP1 aged

0.062 0.28

0.091 0.37

significantly upon aging to 0.28. This change in anisotropy was further probed by measuring the rotational correlation time constant (θ) for these samples. For both fresh and aged ZnSP1, the anisotropy decay follows two-exponential kinetics (Table 4 and Figure 6). For the fresh ZnSP1, the first (θ1) and the second rotational correlation time (θ2) were observed to be 2.2 and 37.5 ns, respectively. However, for the aged ZnSP1, θ1 is decreased to 0.36 ns, while θ2 increases significantly to 152 ns (Table 4). 3.5. Interaction of the NB with Colloidal ZnSP1. In order to assess the difference in surface area of the fresh and aged ZnSP1, different amounts of NB (0.5 × 10−6 to 2.5 × 10−6 mol dm−3) were equilibrated with colloidal ZnSP1. The concentration range of the dye was restricted to 2.5 × 10−6 mol dm−3, as beyond this concentration it is known to undergo dimerization.21 The adsorption isotherms obtained for the adsorption of NB on the fresh and aged ZnSP1 at 303 ± 1 K are shown in Figure 7. From these curves it is apparent that adsorption of the dye on the fresh and aged particle follows Type I and Type IV behavior, respectively. 3.6. Photophysics of ZnSP1 in the Presence of NB. The fluorescence of fresh and aged ZnSP1 was followed for varied concentrations of NB (0.5 × 10−6 to 2.5 × 10−6 mol dm−3). These samples were excited by using 450 nm light radiation, where no change in absorbance of ZnSP1 was observed upon the addition of dye up to 2.5 × 10−6 mol dm−3 (Figure S2). The excitation of these samples caused the quenching of fluorescence due to ZnSP1 in both the red and NIR regions and results in simultaneous appearance of a new band at 670 nm (Figure 8a,b). The new fluorescence peak corresponded to the fluorescence due to the excited NB as was observed in a control experiment (not shown) and has also been reported earlier.22 Interestingly, the intensity of this band increases with

Figure 6. Time-resolved anisotropy decay curves of ZnSP1: fresh (a) and aged (b).

Figure 7. Adsorption isotherms of NB on ZnSP1 curves: fresh (blue line) and aged (green line). Inset: plots of Langmuir adsorption isotherm for fresh (blue line) and aged (green line).

Table 4. Time Resolved Anisotropy Data of Fresh and Aged ZnSP1 (λex = 635 nm; λfl = 770 nm) component 1

component 2

aged (days)

θ1 (ns)

emission %

θ2 (ns)

emission %

χ2

ZnSP1 fresh ZnSP1 aged

2.2 (0.06) 0.36 (0.37)

5.24 0.82

37.5 (0.06) 152 (0.11)

94.76 99.2

1.10 1.09

Values in brackets are pre-exponential factors corresponding to respective τ. 6319

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Figure 8. Fluorescence spectra of fresh (a) and aged (b) ZnSP1 (λex = 450 nm) in the presence of different amounts of NB.

Figure 9. Effect of addition of NB on fluorescence lifetime decay of ZnSP1: (a) λex = 450 nm, λfl = 770 nm; (b) λex = 450 nm, λfl = 670 nm.

beyond 740 nm as a function of [Zn2+] (1.0 × 10−5 to 10.0 × 10−5 mol dm−3), suggesting that it originates possibly by the interaction of Zn2+ with PbSe. It suggests that the addition of Zn2+ induces the additional transitions in the PbSe possibly by creating different surface states and results in its conversion from direct to indirect band gap semiconductor. An increase in the absorption beyond 740 nm (Figure 1a) is understood to be the phonon-assisted optical transitions induced by the added Zn2+. These states might be facilitating the phonon-mediated relaxation to populate the deeper traps responsible for enhanced NIR fluorescence. A scheme representing the occurrence of these processes is given in Figure 10. A further increase in [Zn2+] (15.0 × 10−5 mol dm−3), however, caused a decrease in the absorption coefficient in the entire recorded absorption range (Figure 1a,b). A possibility that these optical and fluorescence changes might have occurred due to the interaction of Zn2+/Pb2+ with RNA alone was probed in a control experiment by recording the absorption spectra of ZnSB1, which has been reported to produce slightly porous structure and is changed to yield folded nanowires upon aging18 (Figure S4). These nanostructures did not show any absorption beyond 350 nm. It clearly reveals that the binding of RNA to PbSe in the presence of Zn2+/Pb2+ brings a change in the morphology to yield honeycomb-like porous nanostructures and is responsible for the observed optical and photophysical behavior of ZnSP1. In fact, the presence of Zn2+ cations induces the folding of RNA23 in RNA-mediated Zn2+/PbSe nano-

increasing the concentration of dye exhibiting an isoemissive point at 710 nm. For the fresh sample, the quenching of fluorescence monitored in the red region both by using steady state and lifetime measurements followed the Stern−Volmer relationship (Figure S3 A and B; Figure 9; Table S2), from which the value of kQ was estimated to be (4.0 ± 0.4) × 1013 mol−1 dm3 s−1. For the fresh ZnSP1, the fluorescence lifetime at 670 nm, where the free NB showed the fluorescence, also decreased with increasing dye (Table S2). At relatively higher concentrations of NB, the fluorescence lifetime is decreased significantly, approaching the lifetime of free NB. In the case of aged ZnSP1, this became more apparent, as the lifetime at higher concentrations of NB (>1.5 × 10−6 mol dm−3) was very similar to that recorded by us and reported for pure NB (0.43 ns) (Table S3). It evidently suggests that NB supported on the aged ZnSP1 could be sensitized more effectively. In order to find out the reactivity of the excited dye with ZnSP1, in a control experiment, the excitation of dye was carried out in the presence of varied amounts of ZnSP1 (0.6 × 10−6 to 2.4 × 10−6 mol dm−3). It did not cause any quenching of fluorescence due to dye, suggesting the absence of injection of electrons from the dye into the nanoparticles in the excited state.

4. DISCUSSION In the optical absorption of RNA-mediated Zn2+/PbSe, the excitonic absorption due to PbSe becomes increasingly less prominent associated with a slight increase in the absorption 6320

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1a) along with a decrease in fluorescence lifetime (Table 1). A decrease in fluorescence lifetime of the first component despite an increase in percentage emission (Table 1) indicates that the nonradiative traps dominate the kinetics. On the contrary, for the aged ZnSP1, the observation of relatively large blue-shifted fluorescence bands in both the red and NIR regions associated with a decrease in fluorescence lifetimes also suggests the participation of nonradiative traps in controlling the kinetics of these processes. This is also evidenced by a tremendous increase in the population of shallower traps for the aged ZnSP1 (57.3%) compared to that of the fresh ZnSP1 (16.5%) (Table 1). A drastic increase in shallow traps in the aged particles might be considered due to the prominent interaction of the Zn(OH)2 species in ZnSP1 upon aging,18 the presence of which was also seen by the XRD and SAED analyses of the aged ZnSP1. An increase in lifetime in the NIR region is corroborated with an increase in the fluorescence intensity in the NIR range (Figure 1b). These changes in fluorescence behavior can be understood by the rapid relaxation of shallowly trapped charge carriers involved in nonradiative transitions to populate the deeper traps now acting as a radiative center to enhance NIR fluorescence. The rapid relaxation is also evidenced by the decrease in the fluorescence lifetimes due to different components in the visible range. A decrease in NIR fluorescence and fluorescence lifetime associated with a blueshifted fluorescence upon aging possibly arises by the confinement of the charge carriers in the porous honeycomblike structure. A significantly higher value of rotational correlation time (θ2) for the aged ZnSP1 can be ascribed to the changed morphology of these nanostructures from simple porous building blocks, earlier seen for the fresh sample, to honeycomb-like 3D superstructures (Figure S1), which would have caused an increase in the anisotropy (Figure 6a; Tables 3 and 4). The porosity of honeycomb structure obtained upon aging of ZnSP1 is also supported by the increased extent of adsorption of NB and higher binding constant for the aged sample as compared to that of fresh ZnSP1 (Figure 7). The photophysics of ZnSP1 observed in the presence of NB suggests that the excited PbSe causes the energy transfer to NB, which results in the quenching of its fluorescence associated with the appearance of a new fluorescence band arising from the excited NB. An examination of the absorption spectrum of NB and the fluorescence spectrum of ZnSP1 (Figure S5) shows an overlapping region in the wavelength range of 640−715 nm. This spectral overlap suggests the possibility of fluorescence resonance energy transfer (FRET), which could become possible if excited PbSe can act as a donor and the NB as an acceptor. The excitation of ZnSP1 shows the quenching of ZnSP1 upon increasing addition of NB, but the quenching of fluorescence is associated with a development of a new band at higher energy, which corresponds to the fluorescence band arising from the excited NB. However, the emission at higher energy indicates the absence of FRET. It is likely to have arisen because of the production of multiexciton per single photon upon excitation of PbSe in ZnSP1 at higher energies (2.755 eV) greater than the bandgap energy (0.278 eV) as shown below in eq i. This aspect has earlier been demonstrated in IV− VI semiconductors by a number of workers.24−26 Moreover, multiple exciton generation with higher charge carrier densities may create higher-energy states,27 which might transfer energy to the adsorbed dye, NB having higher standard redox potential

Figure 10. Schematic representation of PbSe in the absence of Zn2+ ions (left), depicting direct band gap transitions, and in the presence of Zn2+ ions (right), depicting phonon-assisted transitions.

particles due to negative charge on RNA. Under the optimized experimental conditions, the binding of Zn2+ to RNA influences the architecture of Zn2+/PbSe nanoparticles to form a building block, which produces the honeycomb-like porous morphology in the process of self-assembly. Based on the optical and photophysical behavior observed for ZnSP1, it is thus evident that RNA plays a dual role, i.e., in stabilizing PbSe as well as bringing a change in the morphology of Zn2+/ PbSe nanostructures. However, this behavior is quite in contrast to the presence of Mg2+ containing RNA-mediated PbSe, in which the presence of Mg2+ resulted in increased oscillator strength in PbSe, enhancing the absorption coefficient in both the visible and NIR range.16 The difference in the two systems can be understood due to the binding strength and nature of the interactions of these metal ions with the template in the presence of PbSe. A change in the nature of PbSe from direct to indirect band semiconductor is also suggested by the fluorescence spectroscopy, where the band gap fluorescence in the red region due to PbSe is diminished gradually with increasing addition of Zn2+. However, it simultaneously results in an increase in the NIR fluorescence up to 10 × 10−5 mol dm−3 of Zn2+. This increase in fluorescence at lower energy can also be considered to be assisted by the phonon. It might have occurred because of rapid relaxation of charge carriers into the deeper traps. A decrease in the intensity of fluorescence at higher [Zn2+] (15 × 10−5 mol dm−3) is understood by its lower absorption coefficient (Figure 1b). Thus, an amount of 10 × 10−5 mol dm−3 of Zn2+ results in the highest fluorescence in the NIR region. The addition of Zn2+ changes the photophysics of PbSe in a complex scheme as is revealed by the fluorescence lifetime measurements in the red and NIR regions (Tables 1 and 2). An increasing addition of Zn2+ causes a decrease in fluorescence lifetime in the red region and a simultaneous increase in the NIR regions up to 10 × 10−5 mol dm−3 of Zn2+, and, thereafter, it is reduced in both the red and NIR regions (Tables 1 and 2). The variation in quantum efficiency of fluorescence can be understood by a distribution of both the radiative and nonradiative traps. An increasing addition of Zn2+ creates relatively shallower traps involved in nonradiative recombination, resulting in a decrease in fluorescence intensity (Figure 6321

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The Journal of Physical Chemistry C (−0.12),28 before they could annihilate (eq ii). The excited NB then emits at higher energy (670 nm; 1.85 eV) than that of PbSe (806 nm; 1.53 eV).

tissues in the NIR region, where the tissues do not absorb, and in the areas of biomedical research.10,30

■

hv

Zn 2 +/(PbSe)n → Zn 2 +/(PbSe)n (e− − h+)x

ASSOCIATED CONTENT

* Supporting Information

(i)

S

Aged ZnSP1 FESEM image, Elemental mapping, 2D and 3D AFM images, roughness histogram, TEM image, SAED pattern, and EDAX spectrum (Figure S1). Absorption spectra of ZnSP1 with varied amount of NB (Figure S2), Stern Volmer plots (Figure S3), optical absorption spectra of SB, fresh and aged ZnSB1, ZnSP1 (Figure S4), and absorption spectrum of NB and fluorescence spectrum of ZnSP1, showing an overlapping region (Figure S5). Analysis of depth of traps at 770 nm (Table S1A) and 1000 nm (Table S1B), fluorescence lifetimes of fresh ZnSP1 with varied concentration of NB (Table S2), and fluorescence lifetimes of aged ZnSP1 with varied concentration of NB (Table S3). This material is available free of charge via the Internet at http://pubs.acs.org.

Zn 2 +/(PbSe)n (e− − h+)y + NB → Zn 2 +/(PbSe)n + NB* (ii)

where n is the agglomeration number, x > 1, and y > x. Fluorescence lifetime measurements recorded at 770 and 670 nm show an interesting variation (Table S2). The fluorescence at 770 nm is quenched following the Stern−Volmer relationship (Figure S3A,B), but the fluorescence lifetime at 670 nm decreases relatively more rapidly from 15.9 to 2.5 ns and 2.1 to 0.5 ns for the fresh and aged sample, respectively, suggesting that the contribution arising from the emission due to excited NB dominates the decay process. Under identical experimental conditions, the fluorescence lifetime for pure NB was found to be very similar (0.43 ns). The excitation of ZnSP1 by light energy greater than their band gap energy is expected to result in charge carrier annihilation/nonradiative recombination, causing a decrease in the emission intensity. This is also reflected by a decrease in fluorescence lifetime recorded at 770 nm by using 450 nm exciting wavelength (129 ns) as compared to that of observed by using 635 nm exciting wavelength (208 ns). However, in the presence of adsorbed NB on the particle, the exciton−exciton interaction will be reduced, and it is likely to transfer energy to the adsorbed NB. That is the case in the present system in which the energy is transferred from excitonic states of ZnSP1 to the adsorbed NB result in the emission from excited NB. In a control experiment, we have ruled out the transfer of e− from CB of ZnSP1 to the adsorbed NB as was indicated by the absence of any new product upon prolonged irradiation with 450 nm.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] or [email protected]. Phone: +91-1332-285799. Fax: +91-1332-273560. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS K.G. is thankful to CSIR, New Delhi, for the award of JRF. Thanks are also due to the Heads, IIC, and Centre of Nanotechnology, IIT, Roorkee, for providing us the facilities of Single Photon Counter and optical measurements, respectively.

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5. CONCLUSIONS The increasing addition of Zn2+ (1.0 × 10−5 mol dm−3 to 15 × 10−5 mol dm−3) transforms the nature of PbSe from direct to indirect bandgap semiconductor associated with a decrease in the bandgap fluorescence and a simultaneous increase in the NIR fluorescence. A decrease in fluorescence lifetime in the red region and an increase in the NIR region also supported these findings. The observed changes in optical and photophysical behavior can be correlated to the regular alteration in the morphology of these nanostructures from nanoparticles to porous honeycombs. The best photophysical features of PbSe corresponded to the concentration of Zn2+, which produces the honeycomb-like structure. These nanostructures depicted a wide ranging optical absorption and fluorescence band covering the UV−vis−NIR range (200- 1125 nm) and red and NIR regions, respectively. The increase in fluorescence at lower energy in the NIR region is considered to arise due to phononassisted fluorescence. For the optimized sample, a change in morphology from nanoparticles to honeycomb-like structure for ZnSP1 results in increased adsorption for NB, leading to effective energy transfer, suggesting their usage for sensitization. A fairly high NIR lifetime ranging from about 15.11 to 17.60 ns compared to those of organic fluorescent dyes (