8-9th August, 2008 School of Materials Science & Nanotechnology,Jadavpur University
Synthesis and Spectral studies of ZnS/Dendrimer Nanocomposites: Tunability of surface charge and luminescence anisotropy
Srabanti Ghosh, Amiya Priyam and Abhijit Saha*
UGC-DAE Consortium for Scientific Research, Kolkata Centre,
III/LB-8, Bidhannagar, Kolkata -700098, India
*Corresponding author, e-mail: abhijit@alpha.iuc.res.in
Phone: +91-33-23351866 Fax: +91-33-23357008
Abstract:
Highly water-soluble, biocompatible, fluorescent, ZnS/Dendrimer nanocomposites have been synthesized, for the first time, with amino-, carboxyl- or hydroxyl-terminated PAMAM dendrimer. Average size of ZnS NPs within the dendrimer matrix was found to be in the range of 2.2-3.1 nm as estimated by the optical method and transmission electron microscopy. ZnS NPs with high monodispersity and photoluminescence efficiency was obtained by tuning various experimental parameters. An eight-fold increase in supersaturation caused a sharp focusing of size distribution by 38% and photoluminescence quantum efficiency (PLQE) also increases from 1.3% to 4.2%. The ZnS/Dendrimer nanocomposites display constant positive polarized emission with the anisotropy value of 0.2. The anisotropy value further increases as the particle size of ZnS NPs increases and it also depends on surface functionality of the dendrimer molecule. It is also demonstrated that photoluminescence and surface charge of these nanocomposites are governed by the surface functionality of the dendrimer molecule.
1. Introduction:
During the past decade, the movement towards nanodimensions in many areas of technology aroused a huge interest in nanostructurized materials1. A Dendrimer nanocomposite with controlled synthesis of nanoparticles is of immense importance. Dendrimers are three-dimensional, highly branched macromolecule and their highly controllable sizes are determined by the core, extent of branching and nature of end groups2. Dendrimer can be used as a precise nanoreactor for the synthesis of various nanoparticles such as metals, semiconductor, and magnetic oxides3-5.Dendrimer based nanocomposites are novel hybrid materials where inorganic particles are synthesized in situ in dendrimer matrix. As compared to other methods, the dendrimer-mediated synthesis exhibits a greater degree of control with respect to their composition, size and shape, surface functionalities. Nanocomposites containing noble metal NPs of gold, silver and platinum are useful in catalysis, sensors, nanoelectronics and cancer treatment6-7. In recent times, DNC units are being used as fluorescent transfecting agent as well as building blocks for highly ordered nanostructures including self assembled ultrathin multiplayer and smart nano-devices8-9. Although metal/dendrimer nanocomposites are widely explored, there is a very few reports of semiconductor/dendrimer nanocomposites available till date. Dendrimer mediated synthesis of CdS NPs has been carried out by earlier workers10. However, no definite parameters were outlined for controlling the particle characteristics. Very recently, we have reported a synthesis of CdTe/Dendrimer nanocomposites in both aqueous and organic medium11. In view of greater acceptability of Zn in contrast to heavy metals like Cd, Pb, etc. in biological systems, ZnS NPs can be better suited in biomedical applications12.
1
In the present study, we report, for the first time, the role of supersaturation of the initial reaction mixture in directing the size distribution and PLQE of dendrimer mediated semiconductor nanocrystals. ZnS NPs in the dendrimer matrix possess elongated shape and they exhibit polarized emission with high positive anisotropy. In addition, dendrimer molecules impart high surface charge on the ZnS nanoparticles and the nature of the surface charge depends on terminal groups of the dendrimer.
2. Experimental:
Synthesis of ZnS/Dendrimer nanocomposites:
A typical preparation of ZnS/Dendrimer nanocomposites with an initial Zn2+/S2- molar ration of 1:0.5 was as follows: - 5 ml aliquot of Zn2+ stock solution was added to 10 ml of dendrimer stock solution at 50C and then freshly prepared aqueous solution of H2S of known concentration and also aqueous solution of Na2S (1.0 ×10-3 M) were added in stiochiometric amount only in case of studying the effect of sulfide source. Supersaturation for DNC was varied by changing the absolute concentration of each reagent while keeping their ratio constant. The degree of supersaturation corresponding to the 2 mM Zn2+ ions has been denoted as S, which is used for relative scaling of supersaturation. These ZnS/Dendrimer nanocomposites were characterized by UV-Vis spectroscopy, fluorescence spectroscopy, transmission electron microscopy (TEM), selected area electron diffraction (SAED) and zeta potential measurements.
3. Results and discussion:
3.1 TEM:
A typical TEM image for ZnS/Dendrimer nanocomposites is shown in the Figure 1 and the average size of nanoparticles was found to be 2.5 nm as determined from TEM images, which is in good agreement with the size (2.36 nm) determined from the correlation of particle size and optical bandgap. Selected area electron diffraction (SAED) pattern of ZnS nanoparticles is also shown in the inset of Figure 1. The SAED pattern displays bright rings at a distance of 3.1 and 2.7 A° corresponding to 111 and 200 lattice planes of the cubic crystal phase of ZnS.
Fig.1 TEM image of ZnS NPs in dendrimer, Inset: SAED pattern.
3.2 Effect of S2- source:
Figure2 illustrates the photoluminescence and absorption spectra of ZnS/Dendrimer nanocomposites prepared using H2S or Na2S as sulfide ion source. When Na2S was used as sulfide ion source, absorption shoulder was obtained instead of peak, indicating broad
2
size distribution. It is important to note that after few hours, gradual precipitation takes place in the NPs solution prepared using Na2S and consequently absorbance of ZnS NPs was found to decrease with time. But when H2S was used, no such precipitate was formed and particles remained stable for more than three months. Thus, aqueous H2S is preferred to Na2S as sulfide source in synthesis. The PL spectrum consists of peak around 310 nm originated from electron-hole recombination after relaxation (band edge emission) and a strong emission peak around 390 nm due to recombination via surface localized state (the trap state emission). However, the presence of defects due to low temperature synthesis caused trap emission predominates.
300400300400Absorbance (
a.u.)Wavelength(nm)H2S Na2S
Fig 2 (a). Effect of sulfide source on UV-Visible absorption, (b) photoluminescence spectra of ZnS/Dendrimer Nanocomposites.
3.3 Effect of terminal group of the dendrimer:
To examine how dendrimer surface influence the formation of composite particles, ZnS/Dendrimer nanocomposites were prepared by using PAMAM dendrimer with different terminal groups like amino, carboxyl and hydroxyl. On having closer look, in Figure 3 it was observed that a sharp excitonic peak was obtained in case of ZnS/Dendrimer nanocomposites with terminal COOH (ZnS_G4.COOH), whereas a relatively broad spectrum and a broad shoulder were observed with terminal NH2 (ZnS_G4.NH2) and terminal OH (ZnS_G4.OH), respectively. On the other hand, COOH and NH2-terminated dendrimer produced smaller ZnS NPs (2.2 and 2.5 nm, respectively) as compared to OH-terminated dendrimer (3.1 nm). This is probably due to the differential interactions of ZnS NPs with surface group of dendrimers.
Fig.3. Absorption and Photoluminescence of
ZnS/Dendrimer nanocomposites prepared with
varying terminal group of the dendrimer.
300400300400300400Absorbance(a.u.)Wavelength(nm)D-NH2 D-COOH D-OH3003504004505000200400Na2SH2SIntensity(a.u.)Wavelength(nm)
3
3.4 Effect of Supersaturation:
Supersaturation as one of the most important parameters during colloidal synthesis as the evolution of the colloidal nanocrystals involves three steps: supersaturation, nucleation, and growth. It can be easily discerned from Figure 4a that a broad absorption shoulder obtained at 0.5S gets transformed into a peak at 4S indicating narrowing of the size distribution. On going from 0.5S to 4S, the relative percentage distribution decreases from 16% to 10%, which correspond to a sharp focusing of size distribution by about 38%. We interpret this result in terms of the simple correlation between critical size and supersaturation (eq. 1). According to the theoretical model developed by Talapin et al,13 at any given time of growth, there exists a critical radius, which has zero growth rate and is in equilibrium with respect to particle growth. Sln RTV 2γrmcr= (1)
Here, supersaturation is expressed as S = [M]bulk/ C0flat. rcr is the critical radius, [M]bulk the concentration ZnS monomers in the bulk of solution, C0flat the solubility of the bulk material.
3004000.020.040.060.080.100.120.140.160.180.200.220.243004000.020.040.060.080.100.120.140.160.180.203004000.0050.0100.0150.02000250.0300.0350.0400.0450.0500.0550.0600.0653004000.00.20.40.60.81.01.21.4Absorbance(au)Wavelength(nm)0.5SAbsorption(au)Wavelength(nm)2SAbsorbance(au)Wavelength(nm)1SAbsorbance(au)Wavelength(nm)4S30035040045050002004006001. 4S2. 2S3. 1S4. 0.5S4321Intensity(a.u.)Wavelength(nm)
Fig 4(a) Absorption and (b) Luminescence spectra of ZnS NPs synthesized at different degrees of supersaturation. (Synthesis Temp: 5°C, molar ratio of Zn2+: S2- 1:0.5, pH 6.5)
The critical radius (rcr) would be smallest for the NP-solution with highest degree of supersaturation. If this critical size happens to be smaller than the smallest particle present in the ensemble, all particles will show positive growth rate and smaller crystals within the ensemble grow faster than the larger ones leading to focusing of size distribution. For an 8 times change in the degree of supersaturation (0.5S→4S), the PLQE of ZnS NPs goes up by 70% only (1.3% → 4.2%). The growth rate of the NPs decreases with increasing supersaturation which may cause the enhancement of PLQE as slower growth rate leads to lesser defect density15. The enhanced luminescence due to increased supersaturation can have strong implications in the perspective of bio-sensing and bio-imaging.
3.5 Luminescence Anisotropy:
It is quite interesting to note that ZnS NPs in the dendrimer matrix exhibited polarized emission with positive anisotropy. Since most of the NPs are thought to be spherical, the emission is not expected to display any useful polarization. So, the polarized emission is
4
not a general property of NPs but requires special conditions such as synthetic method, stabilizer etc. As evident from TEM image, ZnS NPs are not spherical but have slightly elongated shape. The high and positive anisotropy observed in ZnS/Dendrimer nanocomposites suggests that the excited state dipole is oriented in a fixed direction within the NPs. Anisotropy values of ZnS/Dendrimer nanocomposites are relatively constant around 0.2 across the longer wavelength region (in the range of 275-295 nm) and decreasing towards both shorter and longer wavelengths. This suggests a careful control of the excitation wavelength is necessary during application. In addition anisotropy values increased linearly with increasing particle size of ZnS (Table 1).
Table 1 Effect of ZnS particle size on anisotropy values and average zeta potential data of ZnS_G4.NH2, ZnS_G4.COOH and ZnS_G4.OH nanocomposites at pH 6.5.
ZnS/Dendrimer
Nanocomposites
ParticleSize
(nm)
Anisotropy
Values
Zeta Poptential
(mV)
2.3
0.15
2.5
0.20
2.65
0.20
2.9
0.30
ZnS_G4.NH2
3.2
0.35
+25.4
ZnS_G4.COOH
2.2
0.12
-36.2
ZnS_G4.OH
3.1
0.07
+5.8
3.6 Zeta potential:
The surface charge of the ZnS/Dendrimer nanocomposites was measured by zeta potential, which depends on terminal groups of the dendrimer (Table 1). Primary amino-terminated ZnS nanocomposites (ZnS_G4.NH2) are positively charged whereas carboxyl-terminated ZnS nanocomposites (ZnS_G4.COOH) carry negative charge because of partial ionization of carboxylic groups and hydroxyl-terminated ZnS nanocomposites (ZnS_G4.OH) possess relatively small positive charge. The positive charge of the ZnS_G4.NH2 further indicates that after the formation of the hybrid nanostructures, the terminal amines of dendrimers are still available, which can be used to link biological ligands for biological application. The effects of pH on the zeta potentials of ZnS_G4.NH2 are illustrated in Figure 5. When pH is varied in the range of 2–3, the zeta potential reaches a maximum of 47 mV at pH 2, indicating high stability of the suspension. With increasing pH, zeta potential decreased to 25.4 mV at pH 6.5. Continuous increase in pH to the basic condition gave rise to a negative zeta potential, with its absolute value being increased. Moreover, when pH was larger than 9, the zeta potential remained almost constant at 42.
24681012-40-200204060Zeta potential (
mV)pH
Fig. 5 Zeta potential of ZnS/Dendrimer nanocomposites
as a function of pH.
5
4. Conclusion:
The present study demonstrates the significance of the role of supersaturation and different surface functionally of the dendrimer molecule in determining the quality of semiconductor/dendrimer nanocomposites. It has been shown that a good control over size distribution can be achieved by deftly manipulating the supersaturation, which provides simple route to narrow down the size distribution thereby obviating the need for any post preparative treatment. One of the salient features of this study is that ZnS NPs within dendrimer matrix display positive polarized emission that could be further used as a biophysical probe. The positive charge of these nanocomposites is thought to induce the spontaneous adsorption to negatively charged cellular membrane, prior to cellular internalization and due to high electrostatic interactions, a wide range of bio-ligands can easily bind to the surface of the nanocomposites. Finally we tend to conclude that dendrimer acts as stabilizers, enhouses and protects the nanoclusters as well as exert a direct influence on the properties of the particle within these semiconductor/nanocomposites.
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