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In situ fabrication of amino acid-polyoxometalate nanoparticle functionalized ultrathin films and relevant electrochemical property study
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Materials Research Bulletin 43 (2008) 2880–2886 www.elsevier.com/locate/matresbu
In situ fabrication of amino acid-polyoxometalate nanoparticle functionalized ultrathin films and relevant electrochemical property study Siheng Li, Enbo Wang *, Chungui Tian, Baodong Mao, Yanli Song, Chunlei Wang, Lin Xu Key Laboratory of Polyoxometalate Science of Ministry of Education, Department of Chemistry, Northeast Normal University, Ren Min Street No. 5268, Changchun, Jilin 130024, PR China Received 21 November 2006; received in revised form 27 October 2007; accepted 18 December 2007 Available online 28 December 2007
Abstract In this paper, amino acid-polyoxometalate (polyoxometalate = H3PMo12O40, H4SiW12O40, and H3PW12O40) nanoparticles functionalized ultrathin polyelectrolyte films were successfully fabricated via Layer-by-Layer (LbL) assembly technique. UV–vis absorption spectrometry was used to investigate the incorporation and reproducible growth process of the amino acid-polyoxometalate nanoparticles in the films. Fourier transform infrared spectroscopy was used to measure the composition of the ultrathin polyelectrolyte films. Transmission electron microscopy was employed to study the morphology and size of the nanoparticles in the films. Electrochemical study of the films shows that the excellent electrochemical property of the polyoxometalate remained. # 2007 Elsevier Ltd. All rights reserved. Keywords: A. Thin films; B. Chemical synthesis; C. Electron microscopy; C. Infrared spectroscopy; D. Electrochemical properties
1. Introduction Nowadays, polyoxometalates (POMs) have attracted more and more attention for the potential application in a variety of fields such as medicine, catalysis, materials science and so on [1–6]. As early as 1998, Klemperer and Wall reviewed and predicted that the POM chemistry would move from solids and solutions to surfaces [7]. Numerous examples have fully supported this idea. For instance, POMs were successfully deposited on metal surfaces, electrode surfaces, and then macroscopic flat quartz surfaces, which quickly made POM-based inorganic–organic composite films attainable [8–10]. However, POM chemistry is still experiencing unparalleled development of rapid synthesis of functional POM-based new compounds and slow development of functional materials and devices [11]. Further exploring and developing new POM-based functional devices is still a challenge for POM chemistry. On the other hand, as the basic units of proteins, amino acids are of great significance in biochemistry and life science. The promising applications of POMs as drugs against AIDS and tumors have aroused worldwide interest in
* Corresponding author. Fax: +86 431 85098787. E-mail addresses: [email protected], [email protected] (E. Wang). 0025-5408/$ – see front matter # 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2007.12.012
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the association between POM species and proteins [12–14]. Numerous works have been done to investigate the interaction between amino acid and POMs [15–17]. Amino acid-polyoxometalate nanotubes were successfully prepared by one-step solid-state chemical reaction at room temperature [18]. The versatile strategy so called LbL assembly technique was reported by Decher in 1991 [19,20] has become one of the most promising techniques because of its simplicity and applicability to prepare composite materials containing functional proteins [21], DNA [22], inorganic species and so on [23,24]. Some recent publications reported construction of multifunctional films containing POMs via LbL assembly technique, which gives a feasible method to further develop such functional multilayer film materials to apply as electrocatalysis, molecular electronics and electrooptical devices [25,26]. To date, polyelectrolyte (PE) multilayer films assembled by LbL technology have been proved versatile for the incorporation of nanocomposites. Stroeve’s group has developed a method of in situ nucleation and growth to synthesize inorganic particles within pre-assembled multilayer films [27–29]. In this paper, in situ construction of amino acid-polyoxometalate nanoparticles functionalized ultrathin PE films were successfully achieved via LbL assembly technique. Size of the nanoparticles embedded in films could be tuned by controlling the number of adsorption and precipitation cycle. Electrochemical study showed that amino acidpolyoxometalate nanoparticles in the multilayer films were stable enough to remain the excellent electrochemical property of the POMs. 2. Experimental 2.1. Chemicals Poly(diallyldimethylammonium chloride) (PDDA, MW 100,000–2,000,000) and poly (styrenesulfonate) sodium salt (PSS, MW 70,000) were purchased from Aldrich and used without further treatment. POM with the component H3PMo12O40(PMo12), H4SiW12O40(SiW12) and H3PW12O40(PW12) was prepared according to the literature [30]. Concentrated HCl, H2SO4, 30% H2O2 and lysine purchased from Beijing Chemicals Co. Ltd. were of reagent grade and used as received. 2.2. Instruments UV–vis absorption spectrometry was carried out on a quartz slide using a 756 PC UV–vis spectrophotometer. The Fourier transform infrared (FTIR) spectra were obtained in the absorbance mode using a Bio-Rad FTS135 spectrophotometer with bare silicon wafer used as the reference. Transmission electron microscopy (TEM) (Hitachi, H-600, operating voltage of 120 kV) was used to characterize size and morphology of the amino acid-polyoxometalate nanoparticles in films. Samples for TEM observation were prepared by depositing the PE films on copper grids. The electrochemical measurements were performed on a CHI 660 electrochemical workstation connected to a digital-586 personal computer by using a conventional three-electrode system. Saturated (KCl) Ag/AgCl electrode was used as the reference electrode, Pt gauze as a counter electrode, and the ITO glass (on one side only) as working electrode. 2.3. Substrates treatment PDDA-PSS multilayer films were fabricated and characterized on quartz slides, silicon wafers and ITO glass, respectively. The solid substrates were treated according to the literature [26,31,32]. First, the quartz slides and single crystal silicon wafers were immersed in ‘‘piranha solution’’ (3:7, v/v 30% H2O2/concentrated H2SO4) at 80 8C for 1 h and rinsed with plenty of doubly deionized water, dried under a nitrogen stream, then immersed in the 70 8C solution of H2O–H2O2–NH4OH (5:1:1 v/v/v) for 30 min and thoroughly washed with deionized water and dried under a nitrogen stream. The ITO glass was dipped in H2SO4–H2O2 solution (7:3, v/v) for a few minutes followed by rinsing with deionized water and drying in a nitrogen stream to form a hydrophilic surface before LbL assembly process [8]. 2.4. Film formation The pH of the lysine aqueous solution (0.1 M) was adjusted with dilute aqueous HCl solution to 2. Typically, the multilayer films were prepared by dipping cleaned substrate in PDDA (2 mg ml1) and PSS (1 mg ml1) aqueous
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Scheme 1. Schematic representation of the fabrication process of the ultrathin polyelectrolyte multilayer films containing lys-POM nanoparticles.
solution for 20 min alternately. The substrate was rinsed with deionized water and dried under a nitrogen stream after each immersion step. Then the PDDA/PSS layer called a bilayer was formed on both sides of the substrate. When desired number of bilayers was achieved, the polyelectrolyte-modified substrate was dipped into POMs (POMs = PMo12, SiW12 or PW12) (0.02 M) and lysine aqueous solution for 1 and 2 min, respectively. Rinsing and immersing with deionized water and drying with N2 were performed after each deposition step. The process described above was called an adsorbing and depositing cycle and was repeated every 20 min. The above procedure resulted in the build-up of multilayer films containing lys-POM nanoparticles that can be described as (PDDA/PSS)m (lys-POM)n, where m is the number of polyelectrolyte bilayer and n stands for the number of adsorbing and depositing cycle. Scheme 1 shows a schematic illustration of LbL assembly of the amino acid-polyoxometalate nanoparticles functionalized ultrathin PE films. 3. Results and discussion Fig. 1 shows the UV–vis spectra of (PDDA-PSS)5.5-(lys-PMo12)n multilayer films with n ranging from 1 to 12. PDDA does not absorb above 200 nm and the absorption band at 225 nm is ascribed to the aromatic group of PSS in the pristine film [31,32]. The two characteristic peaks at 219 and 318 nm owing to the O ! Mo charge transfer (CT)
Fig. 1. UV–vis spectra of (PDDA-PSS)5.5-(lys-PMo12)n films with n = 1–12 on solid quartz substrates. The inset shows the plots of the absorbance values at 225 nm, 318 nm vs. the number of precipitation cycle n.
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Fig. 2. FTIR spectrum of PDDA/PSS polyelectrolyte films containing lys-PMo12 nanoparticles assembled on the silicon wafer.
transition of PMo12 confirmed the incorporation of PMo12 in multilayer films. Moreover, plotting the absorbance of multilayer films containing PMo12 at 225 nm, 318 nm versus reaction cycle n results in nearly straight lines (see inset in Fig. 1). The linear increase suggests that the adsorption of PMo12 in the films and the subsequent precipitation with lysine was a successive and reproducible growth process. The FTIR spectrum is an effective method to reveal the composition of the products. Fig. 2 shows the FTIR spectra of pure phase H3PMo12O40 (Fig. 2(A)) and polyelectrolyte multilayer films containing lys-PMo12 nanoparticles (Fig. 2(B)). Comparing with Fig. 2(A), the absorption bands at 1063, 959, 880, and 805 cm1 shown in Fig. 2(B) are ascribed to the vibration modes of y(P Oa), y(Mo Ot), y(Mo–Ob–Mo) and y(Mo–Oc–Mo) of PMo12, respectively [31,32]. The peaks at 3436 and 1740 cm1are attributed to the vibration modes of (y–NH2) and (y C O) of lysine, respectively. The symmetrical NH3+ stretch is observed to give a less intense peak at 1518 cm1. These results indicate that PMo12 and lysine have been successfully embedded in the films. TEM is employed to investigate the morphology and size of the nanoparticles in the films. The evolution process of the lys-POM nanoparticles was explored in 5.5-bilayer ultrathin PDDA-PSS films (as shown in Fig. 3). Fig. 3a shows the TEM image of lys-PMo12 nanoparticles in the films with n = 3. The spherical nanoparticles are well dispersed in the film matrix with sizes of 180–330 nm. Fig. 3b shows the TEM image of lys-PMo12 nanoparticles incorporated in the ultrathin films with n = 4, indicating that size of the nanoparticles is in the range of 250–380 nm. Increasing n to 8, larger nanoparticles (290–490 nm) were obtained (Fig. 3c). It can be concluded that size of the lys-PMo12
Fig. 3. TEM micrographs of (PDDA-PSS)5.5-(lys-PMo12)n films with n is the cycle number of adsorption and precipitation of PMo12. (a) n = 3; (b) n = 4; (c) n = 8.
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Fig. 4. Cyclic voltammograms of (PDDA-PSS)5.5-(lys-PMo12)6 films on ITO electrode in 1 M H2SO4 solution, (a) at 50 mV s1 and (b) at different scan rates (from inner curve to outer curve: 20, 40, 50, 60, 80, 100, 120, 140, 160, 180, 200 mV s1). Potentials vs. the saturated (KCl) Ag/AgCl electrode. The inset shows the plots of the anodic peak II current against scan rates.
nanoparticles increased continuously with the increase of cycle number. The initially formed lys-PMo12 nanoparticles may be the core for the subsequent growth of the nanoparticles. The electrochemical properties of the multilayer films containing lys-PMo12 nanoparticles were studied on ITO. Fig. 4a shows the cyclic voltammograms (CVs) of the films (PDDA-PSS)5.5-(lys-PMo12)6 in 1 M H2SO4 aqueous solutions with the scan rate = 50 mV s1. Three reversible redox peaks appeared in the potential range from +800 to 200 mV and the mean peak potentials E1/2 = (Epa + Epc)/2 are 446(I), 250(II), 9(III) mV, respectively. The three redox peaks correspond to three consecutive two-electron processes [33–35]. Fig. 4b shows the CVs of the PE films containing lys-PMo12 nanoparticles at different scan rates (20–200 mV s1) in 1 M H2SO4. The plots of the peak (II) current versus scan rates are shown in the inset of Fig. 4b. When the scan rates changed from 20 to 200 mV s1, the peak current increased gradually and the cathodic peak potentials shifted to the negative direction and the corresponding anodic peak potentials shifted to the positive direction. The results indicate that the films containing lysPMo12 nanoparticles present similar electrochemical behavior to that of PMo12 in aqueous solution [38]. In our experiments the current response of the films remained unchanged even after 100 or more scan cycles at a rate of 50 mV s1 with the potential range fixed at the range of 200 to +800 mV. It can be concluded that the lys-POM
Fig. 5. (a) UV–vis spectra of (PDDA-PSS)5.5-(lys-SiW12)n films with n = 0–11 on solid quartz substrates. The inset shows the plots of the absorbance values at 222 nm, 268 nm vs. the number of precipitation cycle n. (b) UV–vis spectra of (PDDA-PSS)5.5-(lys-PW12)n films with n = 0–11 on solid quartz substrates. The inset shows the plots of the absorbance values at 218 nm, 268 nm vs. the number of precipitation cycle n.
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Fig. 6. Cyclic voltammograms of (a) (PDDA-PSS)5.5-(lys-PW12)6 films and (b) (PDDA-PSS)5.5-(lys-SiW12)6 films on ITO electrode in 1 M H2SO4 solution at 50 mV s1. Potentials vs. the saturated (KCl) Ag/AgCl electrode.
nanoparticles incorporated in the multilayer films were stable enough to maintain the electrochemical properties of POMs [39]. Series of experiments indicated that SiW12 and PW12 could also be incorporated into the ultrathin polyelectrolyte films. Formation process of multilayer films with the composition (PDDA-PSS)5.5-(lys-SiW12)n (n from 0 to 11) was monitored by UV–vis (shown in Fig. 5(a)). The absorption peak at 268 nm is due to the CT transition of oxygen to tungsten in SiW12, which also suggests the incorporation of SiW12 into multilayer films. The formation process of multilayer films with the composition (PDDA-PSS)5.5-(lys-PW12)n (n from 1 to 10) was also detected by UV–vis as shown in Fig. 5(b). The characteristic band at 268 nm is due to the CT transition of oxygen to tungsten in SiW12 [36,37], which suggests the incorporation of PW12 into multilayer films. Plotting the absorbance of multilayer films containing lys-SiW12 and lys-PW12 nanoparticles at two characteristic peaks versus reaction cycle n also results in nearly straight lines (see insets in Fig. 5(a) and (b)), which indicated that the process of adsorption of SiW12 and PW12 in the films and the subsequent precipitation with lysine was also successive and reproducible. The electrochemical properties of the films (PDDA-PSS)5.5-(lys-PW12)6 (Fig. 6(a)) and (PDDA-PSS)5.5-(lys-SiW12)6 (Fig. 6(b)) were also studied with the scan rate = 50 mV s1. The mean peak potentials are equal to 36(I), 334(II), 733(III) mV for (PDDA-PSS)5.5-(lys-PW12)6 and 244(I), 499(II), 682(III) mV for (PDDA-PSS)5.5-(lys-SiW12)6. 4. Conclusion In situ construction of amino acid-polyoxometalate nanoparticles functionalized ultrathin films was successfully achieved by the LbL assembly technique. The growth of nanoparticles was successive and the structure of the films was not wrecked from the study of UV–vis and FTIR spectra. The lys-POM nanoparticles incorporated in the multilayer films were stable enough to maintain the excellent electrochemical properties of POMs. Experiments are still in progress to investigate the applicability of amino acid-polyoxometalate nanoparticles functionalized ultrathin polyelectrolyte films subminiature devices in catalysis, biomedicine and so on. Acknowledgment The work was financially supported by the National Natural Science Foundation of China (20371011). References [1] M.T. Pope, A. Mu¨ller, Polyoxometalate Chemistry: From Topology via Self-Assembly to Applications, Kluwer, Dordrecht, 2001. [2] M.T. Pope, A. Mu¨ller, Polyoxometalates: From Platonic Solids to Anti Retroviral Activity, Kluwer, Dordrecht, 1994. [3] M.T. Pope, A. Mu¨ller, Angew. Chem. Int. Ed. Engl. 30 (1991) 34.
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S. Li et al. / Materials Research Bulletin 43 (2008) 2880–2886 S. Mandal, P.R. Selvakannan, R. Pasricha, M. Sastry, J. Am. Chem. Soc. 125 (2003) 8440. S. Mandal, A.B. Mandale, M. Sastry, J. Mater. Chem. 14 (2004) 2868. Z.H. Kang, E.B. Wang, B.D. Mao, Z.M. Su, L. Gao, S.Y. Lian, L. Xu, J. Am. Chem. Soc. 127 (2005) 6534. W.G. Klemperer, C.G. Wall, Chem. Rev. 98 (1998) 297. I. Moriguchi, J.H. Fendler, Chem. Mater. 10 (1998) 2205. Y.H. Feng, Z.G. Han, J. Peng, J. Lu, B. Xue, L. Li, H.Y. Ma, E.B. Wang, Mater. Lett. 60 (2006) 1588. G.G. Gao, L. Xu, W.J. Wang, W.J. An, Y.F. Qiu, Z.Q. Wang, E.B. Wang, J. Phys. Chem. B 109 (2005) 8948. S.Q. Liu, D.G. Kurth, B. Bredenko¨tter, D. Volkmer, J. Am. Chem. Soc. 124 (2002) 12279. D.C. Crans, M. Mahroof-Tahir, O.P. Anderson, M.M. Miller, Inorg. Chem. 33 (1994) 5586. F. Xin, M.T. Pope, J. Am. Chem. Soc. 118 (1996) 7731. Y. Inouve, Y. Tokutake, T. Yoshida, Y. Seto, H. Huiiita, K. Dan, A. Yamamoto, S. Nishiya, T. Yamase, S. Nakakr, Antiviral Res. 20 (1993) 317. M. Sadakane, M.H. Dickman, M.T. Pope, Inorg. Chem. 40 (2001) 2715. U. Kortz, M.G. Savelieff, B.S. Bassil, B. Keita, L. Nadjo, Inorg. Chem. 41 (2002) 783. U. Kortz, M.G. Savelieff, F.Y.A. Ghali, L.M. Khalil, S.A. Maalouf, D.I. Sinno, Angew. Chem. Int. Ed. 41 (2002) 4070. R.Y. Wang, D.Z. Jia, L. Zhang, L. Liu, Z.P. Guo, B.Q. Li, J.X. Wang, Adv. Funct. Mater. 16 (2006) 687. G. Decher, J.D. Hong, J. Schmitt, Thin Solid Film 210 (1992) 831. G. Decher, Science 277 (1997) 1232. Y. Lvov, Z. Lu, J.B. Schenkman, X. Zu, J.F. Rusling, J. Am. Chem. Soc. 120 (1998) 4073. S. Jiang, M. Liu, J. Phys. Chem. B 108 (2004) 2880. X. Zhang, J.C. Shen, Adv. Mater. 11 (1999) 1139. M. Schu¨tte, D.G. Kurth, M.R. Linford, H. Co¨lfen, H. Mo¨hwald, Angew. Chem. Int. Ed. 37 (1998) 2891. F. Caruso, D.G. Kurth, D. Volkmer, M.J. Koop, A. Mu¨ller, Langmuir 14 (1998) 3462. D.G. Kurth, D. Volkmer, M. Ruttorf, B. Richter, A. Mu¨ller, Chem. Mater. 12 (2000) 2829. S. Dante, Z.Z. Hou, S. Risbud, P. Stroeve, Langmuir 15 (1999) 2176. L.Q. Zhang, A.K. Dutta, G. Jarero, P. Stroeve, Langmuir 16 (2000) 7095. P. Stroeve, Nucleation of Nanoparticles in Ultrathin Polymer Films in Encyclopedia of Nanoscience Nanotechnology, Marcell Dekker, New York, 2004, p. 2713. M. Filowitz, R.K.C. Ho, W.G. Klemperer, W. Shum, Inorg. Chem. 18 (1979) 93. Y. Lan, E.B. Wang, Y.H. Song, Z.H. Kang, L. Xin, Z. Li, M.Y. Li, New J. Chem. 29 (2005) 1249. Y. Lan, E.B. Wang, Y.H. Song, Y.L. Song, Z.H. Kang, L. Xu, Z. Li, Polymer 47 (2006) 1480. M. Sadakane, E. Steckhan, Chem. Rev. 98 (1998) 219. B. Wang, R.N. Vyas, S. Shaik, Langmuir 23 (2007) 11120. P.J. Kulesza, M. Chojak, K. Karnicka, K. Miecznikowski, B. Palys, A. Lewera, A. Wieckowski, Chem. Mater. 16 (2004) 4128. K. Unoura, N. Tanaka, Inorg. Chem. 22 (1983) 2963. S.Q. Liu, Z.Y. Tang, Z.X. Wang, Z.Q. Peng, E.K. Wang, S.J. Dong, J. Mater. Chem. 10 (2000) 2727. S.Q. Liu, D. Volkmer, D.G. Kurth, J. Cluster Sci. 14 (2003) 405. M. Jiang, E.B. Wang, G. Wei, L. Xu, Z.H. Kang, Z. Li, New J. Chem. 27 (2003) 1291.
Materials Research Bulletin 43 (2008) 2880–2886 www.elsevier.com/locate/matresbu
In situ fabrication of amino acid-polyoxometalate nanoparticle functionalized ultrathin films and relevant electrochemical property study Siheng Li, Enbo Wang *, Chungui Tian, Baodong Mao, Yanli Song, Chunlei Wang, Lin Xu Key Laboratory of Polyoxometalate Science of Ministry of Education, Department of Chemistry, Northeast Normal University, Ren Min Street No. 5268, Changchun, Jilin 130024, PR China Received 21 November 2006; received in revised form 27 October 2007; accepted 18 December 2007 Available online 28 December 2007
Abstract In this paper, amino acid-polyoxometalate (polyoxometalate = H3PMo12O40, H4SiW12O40, and H3PW12O40) nanoparticles functionalized ultrathin polyelectrolyte films were successfully fabricated via Layer-by-Layer (LbL) assembly technique. UV–vis absorption spectrometry was used to investigate the incorporation and reproducible growth process of the amino acid-polyoxometalate nanoparticles in the films. Fourier transform infrared spectroscopy was used to measure the composition of the ultrathin polyelectrolyte films. Transmission electron microscopy was employed to study the morphology and size of the nanoparticles in the films. Electrochemical study of the films shows that the excellent electrochemical property of the polyoxometalate remained. # 2007 Elsevier Ltd. All rights reserved. Keywords: A. Thin films; B. Chemical synthesis; C. Electron microscopy; C. Infrared spectroscopy; D. Electrochemical properties
1. Introduction Nowadays, polyoxometalates (POMs) have attracted more and more attention for the potential application in a variety of fields such as medicine, catalysis, materials science and so on [1–6]. As early as 1998, Klemperer and Wall reviewed and predicted that the POM chemistry would move from solids and solutions to surfaces [7]. Numerous examples have fully supported this idea. For instance, POMs were successfully deposited on metal surfaces, electrode surfaces, and then macroscopic flat quartz surfaces, which quickly made POM-based inorganic–organic composite films attainable [8–10]. However, POM chemistry is still experiencing unparalleled development of rapid synthesis of functional POM-based new compounds and slow development of functional materials and devices [11]. Further exploring and developing new POM-based functional devices is still a challenge for POM chemistry. On the other hand, as the basic units of proteins, amino acids are of great significance in biochemistry and life science. The promising applications of POMs as drugs against AIDS and tumors have aroused worldwide interest in
* Corresponding author. Fax: +86 431 85098787. E-mail addresses: [email protected], [email protected] (E. Wang). 0025-5408/$ – see front matter # 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2007.12.012
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the association between POM species and proteins [12–14]. Numerous works have been done to investigate the interaction between amino acid and POMs [15–17]. Amino acid-polyoxometalate nanotubes were successfully prepared by one-step solid-state chemical reaction at room temperature [18]. The versatile strategy so called LbL assembly technique was reported by Decher in 1991 [19,20] has become one of the most promising techniques because of its simplicity and applicability to prepare composite materials containing functional proteins [21], DNA [22], inorganic species and so on [23,24]. Some recent publications reported construction of multifunctional films containing POMs via LbL assembly technique, which gives a feasible method to further develop such functional multilayer film materials to apply as electrocatalysis, molecular electronics and electrooptical devices [25,26]. To date, polyelectrolyte (PE) multilayer films assembled by LbL technology have been proved versatile for the incorporation of nanocomposites. Stroeve’s group has developed a method of in situ nucleation and growth to synthesize inorganic particles within pre-assembled multilayer films [27–29]. In this paper, in situ construction of amino acid-polyoxometalate nanoparticles functionalized ultrathin PE films were successfully achieved via LbL assembly technique. Size of the nanoparticles embedded in films could be tuned by controlling the number of adsorption and precipitation cycle. Electrochemical study showed that amino acidpolyoxometalate nanoparticles in the multilayer films were stable enough to remain the excellent electrochemical property of the POMs. 2. Experimental 2.1. Chemicals Poly(diallyldimethylammonium chloride) (PDDA, MW 100,000–2,000,000) and poly (styrenesulfonate) sodium salt (PSS, MW 70,000) were purchased from Aldrich and used without further treatment. POM with the component H3PMo12O40(PMo12), H4SiW12O40(SiW12) and H3PW12O40(PW12) was prepared according to the literature [30]. Concentrated HCl, H2SO4, 30% H2O2 and lysine purchased from Beijing Chemicals Co. Ltd. were of reagent grade and used as received. 2.2. Instruments UV–vis absorption spectrometry was carried out on a quartz slide using a 756 PC UV–vis spectrophotometer. The Fourier transform infrared (FTIR) spectra were obtained in the absorbance mode using a Bio-Rad FTS135 spectrophotometer with bare silicon wafer used as the reference. Transmission electron microscopy (TEM) (Hitachi, H-600, operating voltage of 120 kV) was used to characterize size and morphology of the amino acid-polyoxometalate nanoparticles in films. Samples for TEM observation were prepared by depositing the PE films on copper grids. The electrochemical measurements were performed on a CHI 660 electrochemical workstation connected to a digital-586 personal computer by using a conventional three-electrode system. Saturated (KCl) Ag/AgCl electrode was used as the reference electrode, Pt gauze as a counter electrode, and the ITO glass (on one side only) as working electrode. 2.3. Substrates treatment PDDA-PSS multilayer films were fabricated and characterized on quartz slides, silicon wafers and ITO glass, respectively. The solid substrates were treated according to the literature [26,31,32]. First, the quartz slides and single crystal silicon wafers were immersed in ‘‘piranha solution’’ (3:7, v/v 30% H2O2/concentrated H2SO4) at 80 8C for 1 h and rinsed with plenty of doubly deionized water, dried under a nitrogen stream, then immersed in the 70 8C solution of H2O–H2O2–NH4OH (5:1:1 v/v/v) for 30 min and thoroughly washed with deionized water and dried under a nitrogen stream. The ITO glass was dipped in H2SO4–H2O2 solution (7:3, v/v) for a few minutes followed by rinsing with deionized water and drying in a nitrogen stream to form a hydrophilic surface before LbL assembly process [8]. 2.4. Film formation The pH of the lysine aqueous solution (0.1 M) was adjusted with dilute aqueous HCl solution to 2. Typically, the multilayer films were prepared by dipping cleaned substrate in PDDA (2 mg ml1) and PSS (1 mg ml1) aqueous
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Scheme 1. Schematic representation of the fabrication process of the ultrathin polyelectrolyte multilayer films containing lys-POM nanoparticles.
solution for 20 min alternately. The substrate was rinsed with deionized water and dried under a nitrogen stream after each immersion step. Then the PDDA/PSS layer called a bilayer was formed on both sides of the substrate. When desired number of bilayers was achieved, the polyelectrolyte-modified substrate was dipped into POMs (POMs = PMo12, SiW12 or PW12) (0.02 M) and lysine aqueous solution for 1 and 2 min, respectively. Rinsing and immersing with deionized water and drying with N2 were performed after each deposition step. The process described above was called an adsorbing and depositing cycle and was repeated every 20 min. The above procedure resulted in the build-up of multilayer films containing lys-POM nanoparticles that can be described as (PDDA/PSS)m (lys-POM)n, where m is the number of polyelectrolyte bilayer and n stands for the number of adsorbing and depositing cycle. Scheme 1 shows a schematic illustration of LbL assembly of the amino acid-polyoxometalate nanoparticles functionalized ultrathin PE films. 3. Results and discussion Fig. 1 shows the UV–vis spectra of (PDDA-PSS)5.5-(lys-PMo12)n multilayer films with n ranging from 1 to 12. PDDA does not absorb above 200 nm and the absorption band at 225 nm is ascribed to the aromatic group of PSS in the pristine film [31,32]. The two characteristic peaks at 219 and 318 nm owing to the O ! Mo charge transfer (CT)
Fig. 1. UV–vis spectra of (PDDA-PSS)5.5-(lys-PMo12)n films with n = 1–12 on solid quartz substrates. The inset shows the plots of the absorbance values at 225 nm, 318 nm vs. the number of precipitation cycle n.
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Fig. 2. FTIR spectrum of PDDA/PSS polyelectrolyte films containing lys-PMo12 nanoparticles assembled on the silicon wafer.
transition of PMo12 confirmed the incorporation of PMo12 in multilayer films. Moreover, plotting the absorbance of multilayer films containing PMo12 at 225 nm, 318 nm versus reaction cycle n results in nearly straight lines (see inset in Fig. 1). The linear increase suggests that the adsorption of PMo12 in the films and the subsequent precipitation with lysine was a successive and reproducible growth process. The FTIR spectrum is an effective method to reveal the composition of the products. Fig. 2 shows the FTIR spectra of pure phase H3PMo12O40 (Fig. 2(A)) and polyelectrolyte multilayer films containing lys-PMo12 nanoparticles (Fig. 2(B)). Comparing with Fig. 2(A), the absorption bands at 1063, 959, 880, and 805 cm1 shown in Fig. 2(B) are ascribed to the vibration modes of y(P Oa), y(Mo Ot), y(Mo–Ob–Mo) and y(Mo–Oc–Mo) of PMo12, respectively [31,32]. The peaks at 3436 and 1740 cm1are attributed to the vibration modes of (y–NH2) and (y C O) of lysine, respectively. The symmetrical NH3+ stretch is observed to give a less intense peak at 1518 cm1. These results indicate that PMo12 and lysine have been successfully embedded in the films. TEM is employed to investigate the morphology and size of the nanoparticles in the films. The evolution process of the lys-POM nanoparticles was explored in 5.5-bilayer ultrathin PDDA-PSS films (as shown in Fig. 3). Fig. 3a shows the TEM image of lys-PMo12 nanoparticles in the films with n = 3. The spherical nanoparticles are well dispersed in the film matrix with sizes of 180–330 nm. Fig. 3b shows the TEM image of lys-PMo12 nanoparticles incorporated in the ultrathin films with n = 4, indicating that size of the nanoparticles is in the range of 250–380 nm. Increasing n to 8, larger nanoparticles (290–490 nm) were obtained (Fig. 3c). It can be concluded that size of the lys-PMo12
Fig. 3. TEM micrographs of (PDDA-PSS)5.5-(lys-PMo12)n films with n is the cycle number of adsorption and precipitation of PMo12. (a) n = 3; (b) n = 4; (c) n = 8.
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Fig. 4. Cyclic voltammograms of (PDDA-PSS)5.5-(lys-PMo12)6 films on ITO electrode in 1 M H2SO4 solution, (a) at 50 mV s1 and (b) at different scan rates (from inner curve to outer curve: 20, 40, 50, 60, 80, 100, 120, 140, 160, 180, 200 mV s1). Potentials vs. the saturated (KCl) Ag/AgCl electrode. The inset shows the plots of the anodic peak II current against scan rates.
nanoparticles increased continuously with the increase of cycle number. The initially formed lys-PMo12 nanoparticles may be the core for the subsequent growth of the nanoparticles. The electrochemical properties of the multilayer films containing lys-PMo12 nanoparticles were studied on ITO. Fig. 4a shows the cyclic voltammograms (CVs) of the films (PDDA-PSS)5.5-(lys-PMo12)6 in 1 M H2SO4 aqueous solutions with the scan rate = 50 mV s1. Three reversible redox peaks appeared in the potential range from +800 to 200 mV and the mean peak potentials E1/2 = (Epa + Epc)/2 are 446(I), 250(II), 9(III) mV, respectively. The three redox peaks correspond to three consecutive two-electron processes [33–35]. Fig. 4b shows the CVs of the PE films containing lys-PMo12 nanoparticles at different scan rates (20–200 mV s1) in 1 M H2SO4. The plots of the peak (II) current versus scan rates are shown in the inset of Fig. 4b. When the scan rates changed from 20 to 200 mV s1, the peak current increased gradually and the cathodic peak potentials shifted to the negative direction and the corresponding anodic peak potentials shifted to the positive direction. The results indicate that the films containing lysPMo12 nanoparticles present similar electrochemical behavior to that of PMo12 in aqueous solution [38]. In our experiments the current response of the films remained unchanged even after 100 or more scan cycles at a rate of 50 mV s1 with the potential range fixed at the range of 200 to +800 mV. It can be concluded that the lys-POM
Fig. 5. (a) UV–vis spectra of (PDDA-PSS)5.5-(lys-SiW12)n films with n = 0–11 on solid quartz substrates. The inset shows the plots of the absorbance values at 222 nm, 268 nm vs. the number of precipitation cycle n. (b) UV–vis spectra of (PDDA-PSS)5.5-(lys-PW12)n films with n = 0–11 on solid quartz substrates. The inset shows the plots of the absorbance values at 218 nm, 268 nm vs. the number of precipitation cycle n.
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Fig. 6. Cyclic voltammograms of (a) (PDDA-PSS)5.5-(lys-PW12)6 films and (b) (PDDA-PSS)5.5-(lys-SiW12)6 films on ITO electrode in 1 M H2SO4 solution at 50 mV s1. Potentials vs. the saturated (KCl) Ag/AgCl electrode.
nanoparticles incorporated in the multilayer films were stable enough to maintain the electrochemical properties of POMs [39]. Series of experiments indicated that SiW12 and PW12 could also be incorporated into the ultrathin polyelectrolyte films. Formation process of multilayer films with the composition (PDDA-PSS)5.5-(lys-SiW12)n (n from 0 to 11) was monitored by UV–vis (shown in Fig. 5(a)). The absorption peak at 268 nm is due to the CT transition of oxygen to tungsten in SiW12, which also suggests the incorporation of SiW12 into multilayer films. The formation process of multilayer films with the composition (PDDA-PSS)5.5-(lys-PW12)n (n from 1 to 10) was also detected by UV–vis as shown in Fig. 5(b). The characteristic band at 268 nm is due to the CT transition of oxygen to tungsten in SiW12 [36,37], which suggests the incorporation of PW12 into multilayer films. Plotting the absorbance of multilayer films containing lys-SiW12 and lys-PW12 nanoparticles at two characteristic peaks versus reaction cycle n also results in nearly straight lines (see insets in Fig. 5(a) and (b)), which indicated that the process of adsorption of SiW12 and PW12 in the films and the subsequent precipitation with lysine was also successive and reproducible. The electrochemical properties of the films (PDDA-PSS)5.5-(lys-PW12)6 (Fig. 6(a)) and (PDDA-PSS)5.5-(lys-SiW12)6 (Fig. 6(b)) were also studied with the scan rate = 50 mV s1. The mean peak potentials are equal to 36(I), 334(II), 733(III) mV for (PDDA-PSS)5.5-(lys-PW12)6 and 244(I), 499(II), 682(III) mV for (PDDA-PSS)5.5-(lys-SiW12)6. 4. Conclusion In situ construction of amino acid-polyoxometalate nanoparticles functionalized ultrathin films was successfully achieved by the LbL assembly technique. The growth of nanoparticles was successive and the structure of the films was not wrecked from the study of UV–vis and FTIR spectra. The lys-POM nanoparticles incorporated in the multilayer films were stable enough to maintain the excellent electrochemical properties of POMs. Experiments are still in progress to investigate the applicability of amino acid-polyoxometalate nanoparticles functionalized ultrathin polyelectrolyte films subminiature devices in catalysis, biomedicine and so on. Acknowledgment The work was financially supported by the National Natural Science Foundation of China (20371011). References [1] M.T. Pope, A. Mu¨ller, Polyoxometalate Chemistry: From Topology via Self-Assembly to Applications, Kluwer, Dordrecht, 2001. [2] M.T. Pope, A. Mu¨ller, Polyoxometalates: From Platonic Solids to Anti Retroviral Activity, Kluwer, Dordrecht, 1994. [3] M.T. Pope, A. Mu¨ller, Angew. Chem. Int. Ed. Engl. 30 (1991) 34.
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