Facile Synthesis of Fluorescent Silica-Doped Polyvinylpyrrolidone

Article pubs.acs.org/IECR

Facile Synthesis of Fluorescent Silica-Doped Polyvinylpyrrolidone Composites: From Cross-Linked Composite Film to Core−Shell Nanoparticles Yanjiao Lu, Wantai Yang,* and Meizhen Yin* State Key Laboratory of Chemical Resource Engineering, Key Laboratory of Carbon Fiber and Functional Polymers, Ministry of Education, Beijing University of Chemical Technology, 100029 Beijing, China S Supporting Information *

ABSTRACT: Fluorescent silica-doped polyvinylpyrrolidone (PVP) composites with high optical properties have been successfully prepared in a one-pot synthesis through the incorporation of silica nanoparticles and dye molecules into the crosslinked PVP. Scanning electron microscopy, transmission electron microscopy, and fluorescence spectrometry are used to investigate the morphologies and optical properties of the composites. By adjusting the PVP content and reaction time, fluorescent silica-doped PVP film and fluorescent PVP-covered silica core−shell nanoparticles are obtained without stirring and under magnetic stirring, respectively. Because both the silica nanoparticles and the dye molecules react with ring-opened PVP, the composites exhibit highly stable optical properties. The obtained fluorescent composites may have potential applications in sensing and photovoltaic systems. The facile approach can be extended to the preparation of multifunctional fluorescent PVP composites by introducing other types of oxides.

1. INTRODUCTION To date, the incorporation of inorganic nanoparticles into polymer materials is widely used for various sensing,1 photovoltaic,2 and biomedical applications3,4 and thus has attracted increasing attention. Many methods, such as polymerization,5,6 sol−gel reaction,7 and amphiphilic self-assembly,8 have been applied to prepare the inorganic nanoparticle-doped polymer composite materials with required properties and nanostructures. Fluorescent nanomaterials have attracted great interest over the last few decades because of the increasing demand for efficient photosensitive materials. As one of the important classes of luminescent materials, perylenediimide derivatives (PDIs) possess excellent chemical and thermal stability and outstanding photoelectron properties; thus, they have been widely used in many areas, including organic solar cells,9 chemical sensors,10−12 and biomedicine.13−15 Silica nanoparticles (NPs) have been widely applied in catalyst16 and nanomedicine fields17 because of their excellent biocompatibility and the existence of a high amount of silanol groups on the surface.18 Polyvinylpyrrolidone (PVP) is an amphiphilic polymer, with polar amine and carbonyl groups on one side and nonpolar methylene groups on the other. Thus, the polar groups in PVP can interact with the surfaces of silica NPs.3 Our group previously reported that the ring-opening and selfcross-linking of linear PVP occurred under high temperature and pressure.19 In this work, by using silica NPs and linear PVP as the starting materials and N,N′-bis-(2,6-diisopropylphenyl)1,6,7,12-tetrachloroperylene-3,4,9,10-tetracarboxylicacid diimide (4Cl-PDI) as the fluorophore, fluorescent silica-doped PVP composites (film or core−shell NPs) were prepared in a one-pot synthesis (Scheme 1). When the stirring model was changed and the PVP content and reaction time were adjusted, the morphologies of fluorescent silica-doped PVP composites © 2014 American Chemical Society

can be designed and tuned. The silica NPs and the dye molecules were bound to the ring-opened PVP chains by chemical bonds, leading to the high optical stability of the composites. The obtained fluorescent composites might have potential applications in sensing and photovoltaic systems. The facile method might open a new way for the preparation of multifunctional fluorescent PVP composites based on other oxide particles.

2. EXPERIMENTAL SECTION 2.1. Materials. Polyvinylpyrrolidone (PVP, M = 360000 g/ mol), tetraethyl orthosilicate (TEOS), acetone, ethanol, and ammonia aqueous solutions (25%) were purchased from Beijing Chemical Plant and were used as purchased without further purification. N,N′-bis-(2,6-diisopropylphenyl)-1,6,7,12-tetrachloroperylene-3,4,9,10-tetracarboxylic acid diimide (4Cl-PDI) was synthesized according to the literature.20 2.2. Synthesis of Silica NPs. The monodisperse silica NPs were prepared according to a modified Stöber method.21 First, deionized H2O (11 mL), ammonia aqueous solution (5 mL, 25 wt %), and ethanol (30 mL) were blended for 10 min, and then a mixture of TEOS (4 mL) and ethanol (30 mL) was added dropwise into the above solution at room temperature under magnetic stirring. Upon the addition of TEOS, the clear solution gradually turned opaque because of the formation of a white silica suspension. The solution was continuously stirred for 24 h. The NPs were centrifuged at 10 000 rpm, washed with ethanol and water several times, and then dried in vacuum. Received: Revised: Accepted: Published: 2872

September 27, 2013 January 29, 2014 February 4, 2014 February 4, 2014 dx.doi.org/10.1021/ie403211f | Ind. Eng. Chem. Res. 2014, 53, 2872−2877

Industrial & Engineering Chemistry Research

Article

Scheme 1. Schematic Representation of the Formation of (A) Fluorescent Silica-Doped PVP Film and (B) Fluorescent PVPCovered Silica NPs

2.3. Synthesis of Fluorescent Silica-Doped PVP Film. 50 mg of silica NPs (140 nm diameter), 5 mg of 4Cl-PDI, and 800 mg of PVP were dispersed in the mixture of ethanol (10 mL) and acetone (40 mL) under ultrasonication and a mechanical stirring at room temperature for 30 min. Subsequently the mixture was put into a Teflon-lined stainless steel autoclave with a capacity of 60 mL and maintained at 160 °C for 30 h. Then the autoclave was cooled to room temperature. The obtained film was washed with ethanol several times and saved in 30 mL of ethanol. The reaction mixture was evaporated under reduced pressure to remove the solvent. Then, the residue was washed with acetone and centrifuged at 10 000 rpm and dried under vacuum. The residue

4Cl-PDI that was not incorporated into the PVP composites was 1.8 mg. The efficiency in dye entrapment was 64%, which was calculated by the following equation. 4Cl‐PDI entrapment % =

4Cl‐PDIoriginal − 4Cl‐PDI residue 4Cl‐PDIoriginal (1)

2.4. Synthesis of Fluorescent PVP-Covered Silica NPs. 50 mg of silica NPs (140 nm diameter), 5 mg of 4Cl-PDI, and 400 mg of PVP were dispersed in the mixture of ethanol (10 mL) and acetone (40 mL) under ultrasonication and a mechanical stirring at room temperature for 30 min. Subsequently the 2873

dx.doi.org/10.1021/ie403211f | Ind. Eng. Chem. Res. 2014, 53, 2872−2877


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Figure 1. (A) TEM image of silica NPs. (B) Particle size distribution of silica NPs in deionized water.

silica NPs, and dye molecules as the starting materials (Scheme 1). To produce fluorescent silica-doped PVP film, silica NPs (ca. 140 nm diameter), linear PVP, and 4Cl-PDI were dispersed in the mixed solvent of ethanol and acetone. When the reaction was carried out at 160 °C for 30 h without stirring, the fluorescent silica-doped PVP film was obtained (Scheme 1A). The fluorescent silica-doped PVP film was washed with ethanol and acetone several times until the silica NPs and dye molecules could not be extracted by solvents. Whereas the blank crosslinked PVP film exhibited a light yellow color (Figure S2 of Supporting Information), the obtained fluorescent composite film exhibited an orange color, indicating that the dye was successfully incorporated into the film. To further confirm the composition of the composite film, all the samples of SiO2, PVP, and the obtained composite film were characterized by FTIR (Figure 2). Figure 2B presents the IR spectrum of PVP, in which

mixture was put into a Teflon-lined stainless steel autoclave with a capacity of 60 mL and stirred at 160 °C for 11 h. Then the autoclave was cooled to room temperature. The final product was collected by centrifugation, rinsed with ethanol several times, and dried under vacuum. The efficiency in dye entrapment was 28%, which was calculated by eq 1. 2.5. Characterization. Fluorescence measurements were recorded on a FluoroMax-4 spectro fluorometer (Horiba Jobin Yvon, NJ). Scanning electron microscopy (SEM) images were obtained using a HITACH S-4700 scanning electron microscope. Transmission electron microscopy (TEM) images were obtained on an H-800 (Hitachi) transmission electron microscope at an accelerating voltage of 200 kV. Samples dispersed in solution were cast onto a carbon-coated copper grid. Dynamic light scattering (DLS) was performed on a Zetasizer Nano-ZS (Malvern Instruments, U.K.). The samples were diluted with distilled water before testing. Fourier transform infrared (FTIR) spectra were recorded with a Nicolet-50 DXC FTIR spectrophotometer. Dry samples were prepared as KBr pellets at room temperature. Thermogravimetric (TG) and differential thermogravimetric (DTG) analyses were performed on a netzsch TG 209 C Iris system (Germany). The samples were heated in platinum crucibles under flowing nitrogen atmosphere from 25 to 800 °C, using a heating rate of 10 °C/min.

3. RESULTS AND DISCUSSION 3.1. Synthesis of Silica NPs. The monodisperse silica NPs were prepared according to the modified Stöber method.21 The obtained SiO2 NPs appear as a white powder after drying under vacuum (Figure S1A of Supporting Information). Figure 1A shows a typical TEM image of the prepared silica NPs, which are uniform spheres with a diameter of 140 nm. DLS demonstrates that the average size of SiO2 particle is 149 nm and the polydispersity index (PDI) is 0.019 (Figure 1B), indicating the narrow size distribution of the particles. In addition, the size of the silica NPs could be tuned by changing the concentration of TEOS and NH3·H2O and the reaction time.22 3.2. Synthesis of Fluorescent Silica-Doped PVP Film. In our previous work, cross-linked PVP film with a light yellow color was fabricated and the ring-opening and self-cross-linking mechanism of linear PVP was proposed and well-interpreted.19 Papadimitriou reported the interactions between PVP and silica NPs through the covalent bonding between the carbonyl groups in PVP and the surface hydroxyls of silica NPs.3 By combination of above-mentioned reactions, we designed and prepared fluorescent silica-doped PVP composites by using linear PVP,

Figure 2. FTIR spectra of (A) 4Cl- PDI, (B) PVP, (C) composite film, and (D) SiO2.

the characteristic peaks of CO stretching vibration can be observed at 1660 cm−1. Figure 2D illustrates the IR spectrum of SiO2, which shows a strong peak at 1090 cm−1 due to the bending vibration of Si−O band. Figure 2C shows the IR spectrum of the obtained fluorescent composite film. The absorptions at 1660 and 1090 cm−1 are derived from PVP and silica NPs, indicating the successful fabrication of fluorescent silica-doped PVP film. The dye and silica NPs can not be washed out from the fluorescent silica-doped PVP film by solvent, which demonstrates that the silica NPs and dyes were stably incorporated into the PVP film. It was reported that PVP can interact with the 2874



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Figure 3. TG/DTG of (A) the silica-doped PVP composites and (B) the mixture of silica particles and the blank cross-linked PVP.

Figure 4. (A) Emission spectra of 4Cl-PDI and fluorescent silica-doped PVP films in ethanol (λex = 450 nm). (B) Thin-layer chromatography of the reaction mixture. (a: pure 4Cl-PDI. b: 10 mg of 4Cl-PDI; reaction time (r.t.), 11 h. c: 10 mg of 4Cl-PDI; r.t., 30 h. d: 10 mg of 4Cl-PDI and 50 mg of SiO2; r.t., 30 h. e: 5 mg of 4Cl-PDI and 50 mg of SiO2; r.t., 30 h.)

a series of experiments was performed by altering the reaction time and 4Cl-PDI content. In addition, silica NPs were added into the system to check the effect of the addition of bare silica NPs on the substitution rates. Figure 4 shows the emission spectra of fluorescent silica-doped PVP film and the thin-layer chromatography (TLC) of the reaction mixture under different experimental conditions. When 10 mg of 4Cl-PDI was added in the reaction system and the reaction was carried out for 11 h, the maximum absorption peak of the product mixture was close to that of the original free 4Cl-PDI dye (Figure 4A (b)) and new products with higher polarity were generated simultaneously in the mixture (Figure 4B (b)). When the reaction time was prolonged to 30 h, the maximum absorption peak of the product mixture showed a gradual blue-shift compared with that of free 4Cl-PDI dye (Figure 4A (c)), and a few reactant 4Cl-PDI dye still remained in the mixture (Figure 4B (c)). Interestingly, when both 10 mg of 4Cl-PDI and 50 mg of SiO2 were added into the above reaction system and the reaction was still carried out for 30 h, a blue-shifted emission maximum was observed in the fluorescence spectrum of the product mixture (Figure 4A (d)). At this situation, the dye was completely used up (Figure 4B (d)). The above results suggest that silica NPs accelerate the reaction, as supported by the literature.26 When the amount of 4Cl-PDI dye was reduced to 5 mg, the maximum absorption of the product mixture displayed significant blue-shift (Figure 4A (e)) and the polarity of new products obviously increased (Figure 4B (e)). We presumed that the chlorides in the bay region of

surface of silica NPs because of multiple binding sites located in the long PVP polymer chains.3 Therefore, PVP was originally wrapped on the surface of silica NPs. Because the ring-opening and self-cross-linking reaction of PVP occurred under high temperature and pressure, the reaction system contains −COOH and amine groups that were generated from the ring-opened PVP. Li23 reported the esterification between the silica particle surface and oleic acid at 50−80 °C. Therefore, it could be deduced that the chemical reaction occurred between the hydroxyls on the surface of silica NPs and the carboxyl groups in the ring-opened PVP under higher temperature and pressure. To demonstrate the specific interactions between silica and PVP, TG/DTG analysis was performed. The silica-doped PVP composites and the blank cross-linked PVP film were prepared under the same reaction conditions. As shown in Figure 3, both the silica-doped PVP composites (Figure 3A) and the mixture of silica particles and blank cross-linked PVP film (Figure 3B) degraded at 435 °C. An additional higher degradation temperature (548 °C) for the silica-doped PVP composites was detected (Figure 3A). This suggests that the existence of covalent bonding between the silica and PVP is justified because of the higher thermal stability of the PVP composites. Previously, the substitution reactions between the halogen elements and amines have been reported.19,24,25 Because 4ClPDI contains four chloride atoms in the bay region of perylene, the chloride atoms must react with the generated amines at the ring-opened PVP chains. In order to verify the proposed reaction, 2875



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as described in the Experimental Section (Scheme 1B). The PVP-covered silica NPs were centrifuged and washed with ethanol several times and then dried under vacuum to give the final product as an orange powder (Figure S1B of Supporting Information). The FTIR spectra verified that the produced fluorescent NPs were composed of SiO2, PVP, and 4C1-PDI (Figure 6A). TEM was performed to verify the nanostructure of the composites. As shown in Figure 6B, one can clearly see the core−shell structure of PVP-covered silica NPs. The shell thickness is about 10 nm, suggesting the successful covering of PVP chains on the silica particles. The fluorescent emission spectra of the obtained fluorescent PVP-covered silica NPs and the free 4Cl-PDI were measured (Figure 7). In comparison with free 4Cl-PDI dye, a reasonable

perylene were gradually substituted over time in the reaction system. Although the exact structures of these generated products have not been identified, it clearly showed that new products with higher polarity were produced one by one, indicating the reaction between 4Cl-PDI and PVP occurred gradually. As a result, the dye was covalently attached to the PVP chains and labeled the film with fluorescence. As shown in Figure 4A (e), the final obtained composite film exhibited highly stable optical properties with a reasonable blue shift of the emission maximum. The surface morphology of the fluorescent silica-doped PVP film was observed by SEM. Many microspheres were observed on the surface of the fluorescent silica-doped PVP film (Figure 5). Compared with the size of bare

Figure 5. SEM image of the fluorescent silica-doped PVP film. Figure 7. Fluorescent emission spectra of (a) 4Cl-PDI and (b) fluorescent PVP-covered silica NPs (dispersed in ethanol, λex = 450 nm).

silica NPs, the diameter of the microspheres on the film surface exhibited a slight growth, which was attributed to the encapsulation of PVP. Because PVP chains reacted with different silica NPs as well as 4Cl-PDI dye, the interface between the microspheres was inconspicuous. The fluorescence intensity and the surface morphology of the film remained unchanged under natural light even after 14 days of storage, supporting the stable incorporation of silica NPs and dye molecules into the crosslinked PVP films by chemical reactions. Consequently, the fluorescent silica-doped PVP film has been successfully prepared in a one-pot synthesis without stirring. 3.3. Synthesis of Fluorescent PVP-Covered Silica NPs. The fluorescent PVP-covered silica NPs were obtained under stirring by decreasing the amount of PVP and the reaction time,

blue-shifted emission peak of fluorescent composites was observed, which was explained by the substitute reaction between 4Cl-PDI and PVP chains. The fluorescence intensity and also the core−shell nanostructure of the composites remained unchanged under natural light even after 2 weeks of storage as confirmed by optical and TEM analyses, suggesting the high photo- and chemical stability. This observation indicated that the silica NPs and the dye molecules were stably encapsulated in cross-linked PVP chains. Consequently, fluorescent PVP-covered silica NPs have been successfully synthesized under magnetic stirring.

Figure 6. (A) FTIR spectra of (a) 4Cl-PDI, (b) PVP, (c) composites, and (d) SiO2. (B) TEM image of the fluorescent PVP-covered silica NPs. 2876



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(6) Liu, B.; Yang, F.-k.; Liu, G.-y. Synthesis of CdS/SiO2/polymer trilayer fluorescent nanospheres with functional polymer shells. Chin. J. Polym. Sci. 2012, 30, 359. (7) Nakajima, H.; Kawano, K. Preparation and evaluation of the rare earth doped nanoparticle SiO2-PVP hybrid thin film by sol-gel method. J. Alloys Compd. 2006, 408, 701. (8) Li, Y.; Jiang, Y.; Liu, F.; Ren, F.; Zhao, G.; Leng, X. Fabrication and characterization of TiO2/whey protein isolate nanocomposite film. Food Hydrocolloids 2011, 25, 1098. (9) Schmidt-Mende, L.; Fechtenkötter, A.; Müllen, K.; Moons, E.; Friend, R.; MacKenzie, J. Self-organized discotic liquid crystals for highefficiency organic photovoltaics. Science 2001, 293, 1119. (10) Zhou, R.; Li, B.; Wu, N.; Gao, G.; You, J.; Lan, J. Cyclenfunctionalized perylenebisimides as sensitive and selective fluorescent sensors for Pb2+ in aqueous solution. Chem. Commun. 2011, 47, 6668. (11) You, S.; Cai, Q.; Müllen, K.; Yang, W.; Yin, M. pH-Sensitive unimolecular fluorescent polymeric micelles: From volume phase transition to optical response. Chem. Commun. 2014, 50, 823. (12) Yin, M.; Feng, C.; Shen, J.; Yu, Y.; Xu, Z.; Yang, W.; Knoll, W.; Müllen, K. Dual-Responsive Interaction to Detect DNA on TemplateBased Fluorescent Nanotubes. Small 2011, 7, 1629. (13) Xu, Z.; He, B.; Shen, J.; Yang, W.; Yin, M. Fluorescent watersoluble perylenediimide-cored cationic dendrimers: Synthesis, optical properties, and cell uptake. Chem. Commun. 2013, 49, 3646. (14) He, B.; Chu, Y.; Yin, M.; Müllen, K.; An, C.; Shen, J. Fluorescent Nanoparticle Delivered dsRNA Toward Genetic Control of Insect Pests. Adv. Mater. 2013, 25, 4580. (15) Yin, M.; Shen, J.; Pflugfelder, G. O.; Müllen, K. A Fluorescent Core−Shell Dendritic Macromolecule Specifically Stains the Extracellular Matrix. J. Am. Chem. Soc. 2008, 130, 7806. (16) Kim, T. Y.; Park, D. S.; Choi, Y.; Baek, J.; Park, J. R.; Yi, J. Preparation and characterization of mesoporous Zr-WOx/SiO2 catalysts for the esterification of 1-butanol with acetic acid. J. Mater. Chem. 2012, 22, 10021. (17) Tang, L.; Cheng, J. Nonporous silica nanoparticles for nanomedicine application. Nano Today 2013, 8, 290. (18) Jal, P.; Patel, S.; Mishra, B. Chemical modification of silica surface by immobilization of functional groups for extractive concentration of metal ions. Talanta 2004, 62, 1005. (19) Yin, M.; Ye, Y.; Sun, M.; Kang, N.; Yang, W. Facile one-pot synthesis of a polyvinylpyrrolidone-based self-crosslinked fluorescent film. Macromol. Rapid Commun. 2013, 34, 616. (20) Klok, H. A.; Hernández, J. R.; Becker, S.; Müllen, K. Star-shaped fluorescent polypeptides. J. Polym. Sci., Part A: Polym. Chem. 2001, 39, 1572. (21) Stöber, W.; Fink, A.; Bohn, E. Controlled growth of monodisperse silica spheres in the micron size range. J. Colloid Interface Sci. 1968, 26, 62. (22) Wang, X. D.; Shen, Z. X.; Sang, T.; Cheng, X. B.; Li, M. F.; Chen, L. Y.; Wang, Z. S. Preparation of spherical silica particles by Stöber process with high concentration of tetra-ethyl-orthosilicate. J. Colloid Interface Sci. 2010, 341, 23. (23) Li, Z.; Zhu, Y. Surface-modification of SiO2 nanoparticles with oleic acid. Appl. Surf. Sci. 2003, 211, 315. (24) Bernasconi, C. F.; Rappoport, Z. Recent advances in our mechanistic understanding of SNV reactions. Acc. Chem. Res. 2009, 42, 993. (25) Würthner, F.; Stepanenko, V.; Chen, Z.; Saha-Möller, C. R.; Kocher, N.; Stalke, D. Preparation and characterization of regioisomerically pure 1,7-disubstituted perylene bisimide dyes. J. Org. Chem. 2004, 69, 7933. (26) Srihari, P.; Shyam Sunder Reddy, J.; Bhunia, D. C.; Mandal, S.; Yadav, J. PMA-SiO2: A heterogenous catalyst for O-, S-, and Nnucleophilic substitution reactions of aryl propargyl alcohols. Synth. Commun. 2008, 38, 1448.

4. CONCLUSIONS In summary, we explored a general synthetic method for fabrication of fluorescent silica-doped PVP composites. When the reaction was carried out without stirring, fluorescent silicadoped PVP film was obtained under high temperature and pressure. Fluorescent PVP-covered core−shell hybrid NPs were obtained when the reaction was performed under magnetic stirring. Because the silica NPs and the dye molecules reacted with the ring-opened PVP, both the fluorescent PVP film and the PVP-covered silica NPs exhibited high photo- and chemical stability. The reaction between the hydroxyls on the SiO2 surface and the generated −COOH from the ring-opened PVP contributed to the stable incorporation of silica NPs in the composites. The substitution reaction between the halogen elements in the dye and amines derived from the ring-opened PVP was confirmed by TLC and fluorescence analyses. Finally, fluorescent silica-doped PVP composites (film and core−shell NPs) with high optical properties were successfully prepared in one-pot syntheses. The universal synthesis approach can be applied to the fabrication of multifunctional fluorescent PVP composites by using other types of oxides and chromophores.

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ASSOCIATED CONTENT

S Supporting Information *

Digital photographs of the different products. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Science Foundation of China (21174012, 51103008, and 51221002), the New Century Excellent Talents Award Program from the Ministry of Education of China (NCET-10-0215), and the Doctoral Program of Higher Education Research Fund (20120010110008).

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REFERENCES

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Facile Synthesis of Fluorescent Silica-Doped Polyvinylpyrrolidone

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