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Synthesis and characterization of fluorescent chitosan-ZnO hybrid nanospheres
Materials Science and Engineering B 176 (2011) 458–461
Contents lists available at ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Short communication
Synthesis and characterization of fluorescent chitosan-ZnO hybrid nanospheres Eryun Yan a,∗, Cheng Wang b,∗∗, Shuhong Wang b, Liguo Sun b, Yuwei Wang a, Liquan Fan a, Deqing Zhang a a b
College of Material Science and Engineering, Qiqihar University, Qiqihar 161006, PR China Key Laboratory of Polymer Functional Materials, Heilongjiang University, Harbin 150080, PR China
a r t i c l e
i n f o
Article history: Received 22 September 2010 Received in revised form 29 November 2010 Accepted 10 January 2011 Keywords: Chitosan ZnO QDs Fluorescence Nanospheres Hollow
a b s t r a c t Hybrid hollow nanospheres of chitosan-ZnO (CS-ZnO Nps) were successfully prepared by the in situ growing of ZnO quantum dots (QDs) in an aqueous solution consisting of a cationic polymer CS and an anionic monomer acrylic acid (AA), followed by the polymerization of AA and selectively crosslinking of CS with glutaraldehyde. The as-prepared nanospheres were characterized by transmission electron microscopy (TEM), field-emission scanning electron microscopy (FE-SEM), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), UV–visible spectrometer (UV) and fluorescence spectrophotometer (PL). ZnO QDs were dispersed evenly in the shell of hybrid nanospheres, with its dimension less than 5 nm. These fluorescent CS-ZnO Nps were expected to be simultaneously used as biological fluorescent labeling and a carrier for guest materials. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Recently, photoluminescent semiconductor quantum dots (QDs) have gained particular interest due to their potential applications in such as fluorescent probes in biological staining and diagnostics. Compared with conventional organic fluorophores, the semiconductor nanocrystals have a narrow, tunable, symmetric emission spectrum and are photochemically stable, which seem promising in biological analysis. However, traditional semiconductor QDs with high quantum yield such as CdSe or CdTe are harmful to biological species. Although their surface can be protected by ZnS [1], SiO2 [2] or some other organics [3], the leakage of heavy metal ions is inevitable, which will severely affect the human’s health [4]. Moreover, the water stability is another disadvantage for the QDs to be used in the biological field, since the nanoparticles tend to aggregate or undergo Ostwald ripening due to their high surface energy [5]. Thus, searching for an environment friendly semiconductor QDs with stable fluorescence and water solubility is still a challenge. Among the semiconductor nanomaterials, ZnO as a nontoxic material, involving both semiconducting (band gap: 3.37 eV; exciton binding energy: 60 meV) and piezoelectric properties, is promising for the application in ultraviolet (UV) light emitters, varistors [6,7], transparent electrode [8], surface acoustic wave
∗ Corresponding author. Tel.: +86 45186608038; fax: +86 45186609265. ∗∗ Corresponding author. E-mail addresses: [email protected] (E. Yan), wangc [email protected] (C. Wang). 0921-5107/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2011.01.005
devices [9], piezoelectric transducers [10], light-emitting devices [11], UV photodetector [12], gas sensors [13], solar cells [14] and so on. Even though the research focusing on ZnO goes back many decades, few reports have been concerned about its biological applications. Recently, Xiong et al. prepared aqueous ZnO@polymer nanoparticles with green and yellow emission [15], in which, the QDs can be protected by a copolymer shell. Although being successful, some toxic matters in the synthetic process are inevitable, which should be absolutely avoided in biological uses. Aqueous dispersion of ZnO QDs with blue emission was also successfully prepared [16], but after the long time placement, the aggregation still appeared. In previous study, Ding et al. presented a core-templatefree method to prepare magnetic hollow Fe3 O4 -polymer hybrid nanospheres [17]. The pre-synthesized Fe3 O4 nanoparticles were directly added into a polymer–monomer pair aqueous solution containing a cationic polymer chitosan (CS) and an anionic monomer acrylic acid (AA). Initiation of the polymerization with potassium persulfate (K2 S2 O8 ) led to the formation of Fe3 O4 nanoparticles loaded magnetic hollow CS-PAA nanospheres. According to the method mentioned above, herein, we attempted to fabricate photoluminescent ZnO quantum dots (QDs) embedded in the CS-PAA nanospheres. Differently, the ZnO QDs were in situ generated in the process of heating with CS, AA and oleic acid (OA), since CS can provide OH− ions for the synthesis of ZnO QDs, while OA played an important role in giving the bright luminescence, which was also demonstrated in Fu’s work [16]. After polymerization, ZnO QDs were naturally protected by the hydrophilic CS shell. Because of the large cavity of CS-PAA nanospheres and the photoluminescence originated from the ZnO QDs, these CS-ZnO Nps can be
E. Yan et al. / Materials Science and Engineering B 176 (2011) 458–461
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Fig. 1. Images of CS-ZnO Nps observed from FE-SEM (a), TEM (b and c) and HR-TEM (d).
simultaneously used as biological fluorescent labeling and carrier for guest materials. 2. Experimental 2.1. Materials Chitosan (CS, Mw ≈ 200 kDa) was purchased from Nantong Shuanglin Biological Product Inc. It was refined by dissolving in dilute acetic acid solution, filtered, precipitated with aqueous NaOH, and finally dried in vacuum under the room temperature. The degree of deacetylation was about 90%. K2 S2 O8 was recrystallized from distilled water. Acrylic acid (AA, Shanghai Guanghua Chemical Company) was distilled under reduced pressure in nitrogen atmosphere. Zn(NO3 )2 ·6H2 O was supplied by Shanghai Zhenxin Reagent Factory (China); OA by Chengdu Kelong Chemical Reagent Factory (China); glutaraldehyde (GA) by Shanghai Chemical Reagent Supplied Factory (China), respectively. 2.2. Preparation of CS-ZnO Nps The synthesis procedure of CS-ZnO Nps was based on Refs. [18,16]. Typically, 0.125 g purified CS, a pre-determined amount of Zn(NO3 )2 ·6H2 O and OA were dissolved in an aqueous acrylic acid solution (25 mL; 0.06 g acrylic acid). The mixture was allowed to be stirred at 30–40 ◦ C for 30 min, and then heated to 80 ◦ C, retaining for 2 hrs. In succession, polymerization was initiated by the addition of 0.02 g K2 S2 O8 . When the system appeared opalescent, the reaction was cooled to room temperature immediately. Finally, 0.1 mL GA was added to the mixture to crosslink CS selectively. The product was obtained by ultracentrifugation (Ultra ProTM 80, Du Pont) with 8000 rpm for 10 min, and washed with distilled water for three times. These samples were lyophilized to get dry powder for further characterization. 2.3. Characterization The microstructures of CS-ZnO Nps were observed by transmission electron microscopy (TEM, JEOL TEM-100) and high resolution transmission electron microscopy (HR-TEM, JEOL JEM-
2010). Liquid samples were dropped onto copper grid covered with nitrocellulose membrane and they were dried at room temperature before observation. Field-emission scanning electron microscopy (FE-SEM, Philips) was used to observe the surface morphology of nanospheres. A drop of suspension was placed on a clean silicon wafer and air dried. The wafer was sprayed with a thin layer of gold prior to observation. UV–visible spectra of liquid samples were conducted on a UV-1800 PC spectrometer. FT-IR spectra of samples were obtained on a VECTOR-22 spectrometer. PL spectra were performed on a RF-5301 PC fluorescence spectrometer. The crystalline structure of the nanospheres was analyzed on an X-ray diffractometer (XRD, PX13-010). 3. Results and discussion Colloidal ZnO nanoparticles have been widely prepared by sol–gel route [19,20], but they are unstable in aqueous solution and tend to aggregate or undergo Ostwald ripening owing to their high surface energy [5]. These defects limit its applications in biological fields since materials with good water stability are necessary for bioanalysis. Hence, synthesis of nanostructured ZnO particles stable in aqueous system is highly desirable. Our strategy is to in situ embed ZnO QDs into CS-PAA hollow nanospheres via a core-template-free method in a completely aqueous system. Consequently, ZnO QDs can be protected from water by the shell of nanospheres. In a typical procedure, Zn(NO3 )2 ·6H2 O and OA were dissolved in a solution containing a cationic polymer CS, and an anionic monomer AA. In this system, OA was used as surfactant, which can be complex with ZnO QDs and granted the particles stability in aqueous solution. More importantly, the complex between ZnO QDs and OA would be responsible for the strong fluorescence, which has been demonstrated in previous study [16]. After polymerization, the ZnO QDs were encapsulated by the CS-PAA nanospheres. The microstructures of CS-ZnO Nps were observed by TEM and FE-SEM, presented in Fig. 1. Fig. 1a is the FE-SEM image of the as-prepared CS-ZnO Nps. The nanospheres are regularly spherical in shape with the diameter less than 300 nm. Fig. 1b shows the corresponding TEM image of the same sample, and the morphology of spheres is similar to that observed by FE-SEM. Moreover, a
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E. Yan et al. / Materials Science and Engineering B 176 (2011) 458–461
A
(100)
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100 (101) (102) (110)(103)
50
(112)
0 10
20
30
40
50
60
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Fig. 2. XRD patterns of CS-ZnO Nps.
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B CS-ZnO
CS-PAA
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650
600
550
500
450
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Wavelength (cm-1) Fig. 3. FT-IR spectra of CS-PAA Nps and CS-ZnO Nps (A) and its enlargement part (B).
When the starting amount of Zn(NO3 )2 ·6H2 O was 18.5 mg, the shape of PL spectrum of CS-ZnO Nps was similar to that of CSPAA nanospheres. This revealed that the fluorescence of CS-ZnO Nps was very weak and the emission of ZnO QDs was not prominent, maybe owing to its low content. With the amount of ZnO precursor increased to 54.5 mg, about 3 times than before, the fluorescence of hybrid nanospheres was evidently strengthened. When 2.5 2.0
Absorbance (a.u.)
striking feature seen from the broken spheres in this image is that the contrast between the center and the edge is observed, revealing that the hybrid nanospheres have a hollow interior. Thus, it is thought that the existence of ZnO has not affected the hollow morphology of CS-PAA nanospheres. Fig. 1c provides more evidence for the cavum structures of the hybrid nanospheres. Additionally, it’s clearly seen from the HR-TEM image (Fig. 1d) that ZnO QDs disperse well in the shell of CS-PAA nanospheres and their dimension is from 4 to 5 nm. It is well known that CS is a cationic polymer and OA is an anionic surfactant. When Zn(NO3 )2 ·6H2 O was converted into ZnO, the QDs were surrounded by OA rapidly due to their complex. This step was pivotal for the preparation of resulting hybrid nanospheres. On the one hand, the absorption of OA on the surface of ZnO was responsible for the stability of QDs in the aqueous solution; on the other hand, OA endowed ZnO QDs with high negative charges. The negatively charged QDs reacted with positively charged CS, consequently, with the polymerization of AA, ZnO QDs were encapsulated into the spheres. The presence of ZnO QDs in the CS-PAA nanospheres can be testified by XRD patterns, shown in Fig. 2. All of the diffraction peaks from 30 to 70 degree were indexed to a pure wurtzite structure (JCPDS Care No. 89-1397) with no impurities found, indicating that the hybrid nanospheres contained ZnO nanostructures. According to Scherrer formula, the dimension of ZnO Nps was calculated to be about 4.7 nm, consistent with the data from TEM image. But the patterns showed weak signal-to-noise, which may result from the low content of ZnO. To confirm the composition and chemical structures of the resulting nanospheres, FT-IR spectra of CS-PAA nanospheres and CS-ZnO Nps were performed, presented in Fig. 3. In general, no conspicuous differences were observed in both spectra. The strong peak at 1700 cm−1 was attributed to carboxyl group of PAA and the broad peak between 1500 cm−1 and 1700 cm−1 belonged to amidogen of CS. When the spectra were magnified, several peaks attributing to CS-ZnO Nps appeared between 400 cm−1 and 450 cm−1 , but CS-PAA nanospheres had no obvious peak. It was reported that the characteristic peak of ZnO was around 450 cm−1 [21], hence, we deduced that ZnO QDs were embedded into the CSPAA nanospheres. In addition, the absorption feature at 370 nm in the UV spectrum of CS-ZnO Nps also gave a powerful evidence for the existence of ZnO QDs, similar to the pure ZnO (Fig. 4). Materials to be used as biological labeling must possess strong fluorescence. Herein, PL spectra were utilized to characterize the photoluminescent properties of CS-ZnO Nps, shown in Fig. 5. PL spectrum of CS-PAA nanospheres was conducted as reference.
2500
Wavelength (cm-1)
CS-ZnO
1.5 1.0
CS-PAA
0.5 ZnO 0.0 300
400
500
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Wavelengh (nm) Fig. 4. The UV–vis spectra of CS-PAA nanospheres, CS-ZnO Nps and ZnO.
E. Yan et al. / Materials Science and Engineering B 176 (2011) 458–461
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Acknowledgements
CS-ZnO 92.5
The present study has been supported in part by NSFC (Nos. 20872030, 21074031 and 21010402026), CPDF (20090460922), Program for Young Teachers Scientific Research in Qiqihar University (2010k-M25), Elitist Foundation of Heilongjiang University (Hdtd2010-11), Abroad Person with Ability Foundation of Heilongjiang Province (Nos. 1154H01 and 2010td03), and Youthful Technology Innovation Talent Foundation of Harbin (No. GC09A402). The authors greatly thank Dr. Jian Zhang of Group Information Center (BASF SE) for several suggestions.
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PL intensity (a.u.)
CS-ZnO 54.5 500 400 300 200 100 0
CS-PAA
References
CS-ZnO 18.5
400
461
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Wavelength (nm) Fig. 5. PL spectra of CS-PAA nanospheres and CS-ZnO Nps with different amount of ZnO precursor (ex = 350 nm).
the amount of ZnO precursor was further increased to 92.5 mg, the fluorescence intensity of hybrid was largely enhanced. Moreover, with the increase of ZnO precursor from 18.5 mg to 92.5 mg, the size of ZnO QDs decreased from 4.7 nm to 2.9 nm. Consequently, the PL spectra presented an obvious blue-shift due to the quantum size effect [22]. Therefore, the CS-ZnO Nps with tunable and bright fluorescence were expected to be used as biological fluorescent labeling. 4. Conclusions In conclusion, CS-ZnO Nps were successfully prepared by polymerization method in a completely aqueous solution using Zn(NO3 )2 ·6H2 O as the precursor. The hybrid nanospheres showed obvious hollow structure and their diameter was less than 300 nm. ZnO QDs with their dimension less than 5 nm mainly dispersed in the shell of CS-PAA nanospheres. These fluorescent CS-ZnO Nps were expected to be simultaneously used as biological fluorescent labeling and a carrier for guest materials.
[1] S. Li, M.L. Steigerwald, L.E. Brus, ACS Nano 3 (2009) 1267–1273. [2] A. Wolcott, D. Gerion, M. Visconte, J. Sun, A. Schwartzberg, S. Chen, J.Z. Zhang, J. Phys. Chem. B 110 (2006) 5779–5789. [3] Q. Wang, Y. Kuo, Y. Wang, G. Shin, C. Ruengruglikit, Q. Huang, J. Phys. Chem. B 110 (2006) 16860–16866. [4] T. Jamieson, R. Bakhshi, D. Petrova, R. Pocock, M. Imani, A.M. Seifalian, Biomaterials 28 (2007) 4717–4732. [5] W.Z. Ostwald, Phys. Chem. 37 (1901) 385–387. [6] L.Y. Wang, G.Y. Tang, Z.K. Xu, J. Am. Ceram. Soc. 91 (2008) 3742–3745. [7] S.C. Pillai, J.M. Kelly, D.E. McCormack, R. Ramesh, J. Mater. Chem. 18 (2008) 3926–3932. [8] D.R. Sahu, S.-Y. Lin, J.-L. Huang, Appl. Surf. Sci. 252 (2006) 7509–7514. [9] W.-C. Shih, H.-Y. Su, M.S. Wu, Thin Solid Films 517 (2009) 3378–3381. [10] Q.F. Zhou, C. Sharp, J.M. Cannata, K.K. Shung, G.H. Feng, E.S. Kim, Appl. Phys. Lett. 90 (2007) 113502. [11] W.I. Park, G.-C. Yi, Adv. Mater. 16 (2004) 87–90. [12] H. Kind, H. Yan, B. Messer, M. Law, P. Yang, Adv. Mater. 14 (2002) 158–160. [13] Q. Wan, Q.H. Li, Y.J. Chen, T.H. Wang, X.L. He, J.P. Li, C.L. Lin, Appl. Phys. Lett. 84 (2004) 3654. [14] M. Law, L.E. Greene, J.C. Johnson, R. Saykally, P. Yang, Nat. Mater. 4 (2005) 455–459. [15] H.M. Xiong, Y. Xu, Q.G. Ren, Y.Y. Xia, J. Am. Chem. Soc. 130 (2008) 7522–7523. [16] Y.S. Fu, X.W. Du, S.A. Kulinich, J.S. Qiu, W.J. Qin, R. Li, J. Sun, J. Liu, J. Am. Chem. Soc. 129 (2007) 16029–16033. [17] Y. Ding, Y. Hu, X. Jiang, L. Zhang, C. Yang, Angew. Chem. 116 (2004) 6529–6532. [18] Y. Hu, X. Jiang, Y. Ding, Q. Chen, C. Yang, Adv. Mater. 16 (2004) 933–937. [19] M.S. Tokumoto, S.H. Pulcinelli, C.V. Santilli, V. Briois, J. Phys. Chem. B 107 (2002) 568–574. [20] J. Joo, S.G. Kwon, J.H. Yu, T. Hyeon, Adv. Mater. 17 (2005) 1873–1877. [21] Z.J. Wang, H.M. Zhang, L.G. Zhang, J.S. Yuan, S.G. Yan, C.Y. Wang, Nanotechnology 14 (2003) 11–15. [22] L. Zhang, L. Yin, C. Wang, N. Lin, Y. Qi, D. Xiang, J. Phys. Chem. C 114 (2010) 9651–9658.
Contents lists available at ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Short communication
Synthesis and characterization of fluorescent chitosan-ZnO hybrid nanospheres Eryun Yan a,∗, Cheng Wang b,∗∗, Shuhong Wang b, Liguo Sun b, Yuwei Wang a, Liquan Fan a, Deqing Zhang a a b
College of Material Science and Engineering, Qiqihar University, Qiqihar 161006, PR China Key Laboratory of Polymer Functional Materials, Heilongjiang University, Harbin 150080, PR China
a r t i c l e
i n f o
Article history: Received 22 September 2010 Received in revised form 29 November 2010 Accepted 10 January 2011 Keywords: Chitosan ZnO QDs Fluorescence Nanospheres Hollow
a b s t r a c t Hybrid hollow nanospheres of chitosan-ZnO (CS-ZnO Nps) were successfully prepared by the in situ growing of ZnO quantum dots (QDs) in an aqueous solution consisting of a cationic polymer CS and an anionic monomer acrylic acid (AA), followed by the polymerization of AA and selectively crosslinking of CS with glutaraldehyde. The as-prepared nanospheres were characterized by transmission electron microscopy (TEM), field-emission scanning electron microscopy (FE-SEM), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), UV–visible spectrometer (UV) and fluorescence spectrophotometer (PL). ZnO QDs were dispersed evenly in the shell of hybrid nanospheres, with its dimension less than 5 nm. These fluorescent CS-ZnO Nps were expected to be simultaneously used as biological fluorescent labeling and a carrier for guest materials. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Recently, photoluminescent semiconductor quantum dots (QDs) have gained particular interest due to their potential applications in such as fluorescent probes in biological staining and diagnostics. Compared with conventional organic fluorophores, the semiconductor nanocrystals have a narrow, tunable, symmetric emission spectrum and are photochemically stable, which seem promising in biological analysis. However, traditional semiconductor QDs with high quantum yield such as CdSe or CdTe are harmful to biological species. Although their surface can be protected by ZnS [1], SiO2 [2] or some other organics [3], the leakage of heavy metal ions is inevitable, which will severely affect the human’s health [4]. Moreover, the water stability is another disadvantage for the QDs to be used in the biological field, since the nanoparticles tend to aggregate or undergo Ostwald ripening due to their high surface energy [5]. Thus, searching for an environment friendly semiconductor QDs with stable fluorescence and water solubility is still a challenge. Among the semiconductor nanomaterials, ZnO as a nontoxic material, involving both semiconducting (band gap: 3.37 eV; exciton binding energy: 60 meV) and piezoelectric properties, is promising for the application in ultraviolet (UV) light emitters, varistors [6,7], transparent electrode [8], surface acoustic wave
∗ Corresponding author. Tel.: +86 45186608038; fax: +86 45186609265. ∗∗ Corresponding author. E-mail addresses: [email protected] (E. Yan), wangc [email protected] (C. Wang). 0921-5107/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2011.01.005
devices [9], piezoelectric transducers [10], light-emitting devices [11], UV photodetector [12], gas sensors [13], solar cells [14] and so on. Even though the research focusing on ZnO goes back many decades, few reports have been concerned about its biological applications. Recently, Xiong et al. prepared aqueous ZnO@polymer nanoparticles with green and yellow emission [15], in which, the QDs can be protected by a copolymer shell. Although being successful, some toxic matters in the synthetic process are inevitable, which should be absolutely avoided in biological uses. Aqueous dispersion of ZnO QDs with blue emission was also successfully prepared [16], but after the long time placement, the aggregation still appeared. In previous study, Ding et al. presented a core-templatefree method to prepare magnetic hollow Fe3 O4 -polymer hybrid nanospheres [17]. The pre-synthesized Fe3 O4 nanoparticles were directly added into a polymer–monomer pair aqueous solution containing a cationic polymer chitosan (CS) and an anionic monomer acrylic acid (AA). Initiation of the polymerization with potassium persulfate (K2 S2 O8 ) led to the formation of Fe3 O4 nanoparticles loaded magnetic hollow CS-PAA nanospheres. According to the method mentioned above, herein, we attempted to fabricate photoluminescent ZnO quantum dots (QDs) embedded in the CS-PAA nanospheres. Differently, the ZnO QDs were in situ generated in the process of heating with CS, AA and oleic acid (OA), since CS can provide OH− ions for the synthesis of ZnO QDs, while OA played an important role in giving the bright luminescence, which was also demonstrated in Fu’s work [16]. After polymerization, ZnO QDs were naturally protected by the hydrophilic CS shell. Because of the large cavity of CS-PAA nanospheres and the photoluminescence originated from the ZnO QDs, these CS-ZnO Nps can be
E. Yan et al. / Materials Science and Engineering B 176 (2011) 458–461
459
Fig. 1. Images of CS-ZnO Nps observed from FE-SEM (a), TEM (b and c) and HR-TEM (d).
simultaneously used as biological fluorescent labeling and carrier for guest materials. 2. Experimental 2.1. Materials Chitosan (CS, Mw ≈ 200 kDa) was purchased from Nantong Shuanglin Biological Product Inc. It was refined by dissolving in dilute acetic acid solution, filtered, precipitated with aqueous NaOH, and finally dried in vacuum under the room temperature. The degree of deacetylation was about 90%. K2 S2 O8 was recrystallized from distilled water. Acrylic acid (AA, Shanghai Guanghua Chemical Company) was distilled under reduced pressure in nitrogen atmosphere. Zn(NO3 )2 ·6H2 O was supplied by Shanghai Zhenxin Reagent Factory (China); OA by Chengdu Kelong Chemical Reagent Factory (China); glutaraldehyde (GA) by Shanghai Chemical Reagent Supplied Factory (China), respectively. 2.2. Preparation of CS-ZnO Nps The synthesis procedure of CS-ZnO Nps was based on Refs. [18,16]. Typically, 0.125 g purified CS, a pre-determined amount of Zn(NO3 )2 ·6H2 O and OA were dissolved in an aqueous acrylic acid solution (25 mL; 0.06 g acrylic acid). The mixture was allowed to be stirred at 30–40 ◦ C for 30 min, and then heated to 80 ◦ C, retaining for 2 hrs. In succession, polymerization was initiated by the addition of 0.02 g K2 S2 O8 . When the system appeared opalescent, the reaction was cooled to room temperature immediately. Finally, 0.1 mL GA was added to the mixture to crosslink CS selectively. The product was obtained by ultracentrifugation (Ultra ProTM 80, Du Pont) with 8000 rpm for 10 min, and washed with distilled water for three times. These samples were lyophilized to get dry powder for further characterization. 2.3. Characterization The microstructures of CS-ZnO Nps were observed by transmission electron microscopy (TEM, JEOL TEM-100) and high resolution transmission electron microscopy (HR-TEM, JEOL JEM-
2010). Liquid samples were dropped onto copper grid covered with nitrocellulose membrane and they were dried at room temperature before observation. Field-emission scanning electron microscopy (FE-SEM, Philips) was used to observe the surface morphology of nanospheres. A drop of suspension was placed on a clean silicon wafer and air dried. The wafer was sprayed with a thin layer of gold prior to observation. UV–visible spectra of liquid samples were conducted on a UV-1800 PC spectrometer. FT-IR spectra of samples were obtained on a VECTOR-22 spectrometer. PL spectra were performed on a RF-5301 PC fluorescence spectrometer. The crystalline structure of the nanospheres was analyzed on an X-ray diffractometer (XRD, PX13-010). 3. Results and discussion Colloidal ZnO nanoparticles have been widely prepared by sol–gel route [19,20], but they are unstable in aqueous solution and tend to aggregate or undergo Ostwald ripening owing to their high surface energy [5]. These defects limit its applications in biological fields since materials with good water stability are necessary for bioanalysis. Hence, synthesis of nanostructured ZnO particles stable in aqueous system is highly desirable. Our strategy is to in situ embed ZnO QDs into CS-PAA hollow nanospheres via a core-template-free method in a completely aqueous system. Consequently, ZnO QDs can be protected from water by the shell of nanospheres. In a typical procedure, Zn(NO3 )2 ·6H2 O and OA were dissolved in a solution containing a cationic polymer CS, and an anionic monomer AA. In this system, OA was used as surfactant, which can be complex with ZnO QDs and granted the particles stability in aqueous solution. More importantly, the complex between ZnO QDs and OA would be responsible for the strong fluorescence, which has been demonstrated in previous study [16]. After polymerization, the ZnO QDs were encapsulated by the CS-PAA nanospheres. The microstructures of CS-ZnO Nps were observed by TEM and FE-SEM, presented in Fig. 1. Fig. 1a is the FE-SEM image of the as-prepared CS-ZnO Nps. The nanospheres are regularly spherical in shape with the diameter less than 300 nm. Fig. 1b shows the corresponding TEM image of the same sample, and the morphology of spheres is similar to that observed by FE-SEM. Moreover, a
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E. Yan et al. / Materials Science and Engineering B 176 (2011) 458–461
A
(100)
200
CS-ZnO
Intensity (a.u.)
CS-ZnO (002)
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100 (101) (102) (110)(103)
50
(112)
0 10
20
30
40
50
60
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2Theta (degree)
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Fig. 2. XRD patterns of CS-ZnO Nps.
2000
1500
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B CS-ZnO
CS-PAA
700
650
600
550
500
450
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Wavelength (cm-1) Fig. 3. FT-IR spectra of CS-PAA Nps and CS-ZnO Nps (A) and its enlargement part (B).
When the starting amount of Zn(NO3 )2 ·6H2 O was 18.5 mg, the shape of PL spectrum of CS-ZnO Nps was similar to that of CSPAA nanospheres. This revealed that the fluorescence of CS-ZnO Nps was very weak and the emission of ZnO QDs was not prominent, maybe owing to its low content. With the amount of ZnO precursor increased to 54.5 mg, about 3 times than before, the fluorescence of hybrid nanospheres was evidently strengthened. When 2.5 2.0
Absorbance (a.u.)
striking feature seen from the broken spheres in this image is that the contrast between the center and the edge is observed, revealing that the hybrid nanospheres have a hollow interior. Thus, it is thought that the existence of ZnO has not affected the hollow morphology of CS-PAA nanospheres. Fig. 1c provides more evidence for the cavum structures of the hybrid nanospheres. Additionally, it’s clearly seen from the HR-TEM image (Fig. 1d) that ZnO QDs disperse well in the shell of CS-PAA nanospheres and their dimension is from 4 to 5 nm. It is well known that CS is a cationic polymer and OA is an anionic surfactant. When Zn(NO3 )2 ·6H2 O was converted into ZnO, the QDs were surrounded by OA rapidly due to their complex. This step was pivotal for the preparation of resulting hybrid nanospheres. On the one hand, the absorption of OA on the surface of ZnO was responsible for the stability of QDs in the aqueous solution; on the other hand, OA endowed ZnO QDs with high negative charges. The negatively charged QDs reacted with positively charged CS, consequently, with the polymerization of AA, ZnO QDs were encapsulated into the spheres. The presence of ZnO QDs in the CS-PAA nanospheres can be testified by XRD patterns, shown in Fig. 2. All of the diffraction peaks from 30 to 70 degree were indexed to a pure wurtzite structure (JCPDS Care No. 89-1397) with no impurities found, indicating that the hybrid nanospheres contained ZnO nanostructures. According to Scherrer formula, the dimension of ZnO Nps was calculated to be about 4.7 nm, consistent with the data from TEM image. But the patterns showed weak signal-to-noise, which may result from the low content of ZnO. To confirm the composition and chemical structures of the resulting nanospheres, FT-IR spectra of CS-PAA nanospheres and CS-ZnO Nps were performed, presented in Fig. 3. In general, no conspicuous differences were observed in both spectra. The strong peak at 1700 cm−1 was attributed to carboxyl group of PAA and the broad peak between 1500 cm−1 and 1700 cm−1 belonged to amidogen of CS. When the spectra were magnified, several peaks attributing to CS-ZnO Nps appeared between 400 cm−1 and 450 cm−1 , but CS-PAA nanospheres had no obvious peak. It was reported that the characteristic peak of ZnO was around 450 cm−1 [21], hence, we deduced that ZnO QDs were embedded into the CSPAA nanospheres. In addition, the absorption feature at 370 nm in the UV spectrum of CS-ZnO Nps also gave a powerful evidence for the existence of ZnO QDs, similar to the pure ZnO (Fig. 4). Materials to be used as biological labeling must possess strong fluorescence. Herein, PL spectra were utilized to characterize the photoluminescent properties of CS-ZnO Nps, shown in Fig. 5. PL spectrum of CS-PAA nanospheres was conducted as reference.
2500
Wavelength (cm-1)
CS-ZnO
1.5 1.0
CS-PAA
0.5 ZnO 0.0 300
400
500
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Wavelengh (nm) Fig. 4. The UV–vis spectra of CS-PAA nanospheres, CS-ZnO Nps and ZnO.
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Acknowledgements
CS-ZnO 92.5
The present study has been supported in part by NSFC (Nos. 20872030, 21074031 and 21010402026), CPDF (20090460922), Program for Young Teachers Scientific Research in Qiqihar University (2010k-M25), Elitist Foundation of Heilongjiang University (Hdtd2010-11), Abroad Person with Ability Foundation of Heilongjiang Province (Nos. 1154H01 and 2010td03), and Youthful Technology Innovation Talent Foundation of Harbin (No. GC09A402). The authors greatly thank Dr. Jian Zhang of Group Information Center (BASF SE) for several suggestions.
600
PL intensity (a.u.)
CS-ZnO 54.5 500 400 300 200 100 0
CS-PAA
References
CS-ZnO 18.5
400
461
500
600
Wavelength (nm) Fig. 5. PL spectra of CS-PAA nanospheres and CS-ZnO Nps with different amount of ZnO precursor (ex = 350 nm).
the amount of ZnO precursor was further increased to 92.5 mg, the fluorescence intensity of hybrid was largely enhanced. Moreover, with the increase of ZnO precursor from 18.5 mg to 92.5 mg, the size of ZnO QDs decreased from 4.7 nm to 2.9 nm. Consequently, the PL spectra presented an obvious blue-shift due to the quantum size effect [22]. Therefore, the CS-ZnO Nps with tunable and bright fluorescence were expected to be used as biological fluorescent labeling. 4. Conclusions In conclusion, CS-ZnO Nps were successfully prepared by polymerization method in a completely aqueous solution using Zn(NO3 )2 ·6H2 O as the precursor. The hybrid nanospheres showed obvious hollow structure and their diameter was less than 300 nm. ZnO QDs with their dimension less than 5 nm mainly dispersed in the shell of CS-PAA nanospheres. These fluorescent CS-ZnO Nps were expected to be simultaneously used as biological fluorescent labeling and a carrier for guest materials.
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