Green synthesis and characterization of polymer-stabilized silver nanoparticles

Colloids and Surfaces B: Biointerfaces 73 (2009) 185–191

Contents lists available at ScienceDirect

Colloids and Surfaces B: Biointerfaces journal homepage: www.elsevier.com/locate/colsurfb

Green synthesis and characterization of polymer-stabilized silver nanoparticles Iliana Medina-Ramirez a , Sajid Bashir b , Zhiping Luo c , Jingbo Louise Liu c,∗ a

Chemistry Department, Universidad Autonoma de Aguascalientes. Av. Universidad 940C. P., Aguascalientes, Aguascalientes 20100, Mexico Chemical Biology Research Group, Chemistry Department, Texas A&M University-Kingsville, MSC 161, 700 University Boulevard, Kingsville, TX, 78363 USA c Microscopy and Imaging Center, Texas A&M University-College Station, BSBW, 2257 TAMU, College Station, TX 77843, USA b

a r t i c l e

i n f o

Article history: Received 8 May 2009 Accepted 14 May 2009 Available online 23 May 2009 Keywords: Green Chemistry Nanoparticles Microstructure Characterization Electrokinetics

a b s t r a c t Silver nanoparticles (Ag-NPs) were synthesized using a facile green chemistry synthetic route. The reaction occurred at ambient temperature with four reducing agents introduced to obtain nanoscale Ag-NPs. The variables of the green synthetic route, such as acidity, concentration of starting materials, and molar ratio of reactants were optimized. Dispersing agents were employed to prevent Ag-NPs from aggregating. Advanced instrumentation techniques, such as X-ray powder diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), ultraviolet–visible spectroscopy (UV–vis), and phase analysis light scattering technique (ZetaPALS) were applied to characterize the morphology, particle size distribution, elemental composition, and electrokinetic behavior of the Ag-NPs. UV–vis spectra detected the characteristic plasmon at approximately 395–410 nm; and XRD results were indicative of face-centered cubic phase structure of Ag. These particles were found to be monodispersed and highly crystalline, displaying near-spherical appearance, with average particle size of 10.2 nm using citrate or 13.7 nm using ascorbic acid as reductants from particle size analysis by ZetaPALS, respectively. The rapid electrokinetic behavior of the Ag was evaluated using zetapotential (from −40 to −42 mV), which was highly dependant on nanoparticle acidity and particle size. The current research opens a new avenue for the green fabrication of nanomaterials (including variables optimization and aggregation prevention), and functionalization in the field of nanocatalysis, disinfection, and electronics. © 2009 Elsevier B.V. All rights reserved.

1. Introduction The study of metal nanosized systems has attracted considerable interest in recent years. Metal nanoparticles have a number of applications, from electronics and catalysis to pharmaceutical and medical diagnosis [1–4]. Silver compounds have an array of properties that could be tuned or enhanced through control of size, surface chemistry and morphology at nanoscale dimensions [5,6]. A better understanding of the properties of silver nanoparticles (Ag-NPs) has led to the discovery of numerous applications using this material [7,8]. Silver is also the most ‘popular’ catalyst for the oxidation of ethylene to ethylene oxide and methanol to formaldehyde [9]. It has the highest electrical and thermal conductivity among metals, making it a preferred material for electrical contacts and an additive for conducting adhesives [10]. More recently, Ag-NPs have locally amplify light by 10–100 times, leading to surface-enhanced Raman scattering (SERS), with enhancement factors on the order of 104 to 106 [11]. Silver products have long been known to have strong inhibitory and bactericidal effects, as

∗ Corresponding author. Tel.: +1 361 593 2919; fax: +1 361 593 3597. E-mail addresses: [email protected] (I. Medina-Ramirez), [email protected] (S. Bashir), [email protected] (Z. Luo), [email protected] (J.L. Liu). 0927-7765/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfb.2009.05.015

well as a broad spectrum of antimicrobial activities, which has been used for centuries to prevent and treat a variety of diseases, most notably infections [12–14]. Ag-NPs were reported recently as antibacterial agents against Escherichia coli [15], Staphylococcus, methicillin-resistant Staphylococcus epidermis (MRSE) [16], and methicillin-resistant Staphylococcus aureus (MRSA) bacteria [17]. Ag-NPs also exhibit potent cytoprotective activity toward human immunodeficiency virus (HIV) infected cells [18]. Lately, the application of silver nanoparticles as biological labels in neuroblastoma cells was reported [19–22]. Studies on the interactions between Ag-NPs and live cells have been possible due to the intense plasmon-resonant properties of Ag-NPs and the enhanced resolution obtainable with high-illumination systems in physiological solutions [23]. Despite the many applications of Ag-NPs, to date, there is little progress in developing a facile green route for the formation of robust metal nanoparticles and metal nanoparticle–polymer composites [24–26]. Early studies for the production of Ag-NPs were conducted in micellar solutions. The most significant disadvantages of this method are the use of toxic surfactants, organic solvents and ␥-irradiation; besides, the particles in micellar solution are polydispersed [27–29]. Later, the preparation of Ag-NPs with water-soluble polymers, gelatin (P-11 food grade), poly(vinyl alcohol) and methylhydroxyethyl cellulose was reported [30]. This report indicated that

186

I. Medina-Ramirez et al. / Colloids and Surfaces B: Biointerfaces 73 (2009) 185–191

the use of hydroxyl-containing polymers as stabilizers makes it possible to control and to vary the mean diameter and the character of nanoparticle size distribution [31–33]. Over the past decade there has been an increased interest on the topic of “green” chemistry. A comprehensive review regarding greener nanosynthesis was presented by Pastoriza and Zhang [34,35]. Green chemistry methods offer opportunities to design greener nanomaterials and nanomaterial production methods (i.e., safer nanomaterials, reduced environmental impact, waste reduction, and energy efficiency), thus can play a prominent role in guiding the development of nanotechnology to provide the maximum benefit of these products for society and the environment [36,37]. Unfortunately, many of the nanomaterials synthesis or production methods involve use of hazardous chemicals, low material conversions, high energy requirements, and difficult, wasteful purifications; therefore, there is a need to develop greener processes for the production of these materials [38,39]. The number of publications on the preparation of nanomaterials applying the principles of green chemistry, is growing [40–42]. For example, the use of green reducing agents, such as citrate, has been explored for the production of Ag and Au nanoparticles [43,44]. Despite the use of less hazardous reagents and water as reaction media, it was found that the resulting particles have limited stability and poorly defined surface functionality [45]. In this study, we report the results of the effects of various factors on the formation of Ag-NPs. Our results indicate that the formation of stable, monodispersed colloidal solutions of silver nanocomposites and silver nanoparticles can be achieved using green chemistry methods. The materials were prepared in the form of aqueous dispersions using gum Arabic (GA) as stabilizing agent, favoring the formation of small metal particles; metal salts as metal precursor and different reducing agents. The synthesis was carried out at low temperatures (<100 ◦ C). The properties of nanomaterials were characterized by means of Ultraviolet-Visible (UV–vis) spectroscopy, X-ray powder diffraction (XRD), ZetaPALSTM , transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). We believe that the reproducible facile green route will offer new possibilities for the practical application of metal nanoparticles by simplifying the procedure for the production of complex structured nanomaterials (for examples by using Ag–TiO2 ). This green approach has also been applied to the synthesis of other noble (such as gold and platinum) and transition metals (nickel and cobalt) in Liu’s laboratory. 2. Material and methods 2.1. Ag-NPs preparation Silver nitrate (Spectrum, Gardena CA, 99.9%) and doubledistilled water (Milli-Q, Billerica, MA) were used to prepare the aqueous solution of metal salt. Gum Arabic (GA) (Sigma–Aldrich, St. Louis, MO, reagent grade) was used as dispersing agent. Sodium borohydride (Alfa Aesar, Ward Hill, MA, 98%), sodium citrate (Fisher Scientific Company, Pittsburgh, PA, laboratory grade), lascorbic acid (Mallinckrodt, Cincinnati, OH, Analytical Reagent) and dimethylamine borane (DMAB) (TCI America, Wellesley, MA, 95%) were used as reducing agents. All reagents were used as received. Colloidal suspensions of silver nanoparticles (Ag-NPs) were synthesized by the chemical reduction of silver nitrate. The GA aqueous solution (3% (m/v) 50 mL) was stirred for 30 min at 60 ◦ C. Then 3.5 mL of 0.1 M AgNO3 aqueous solution was added under continuous agitation. The mixture was agitated for 15 min to allow the diffusion of metal ions to the pores of dispersing agent. Reducing agent solution was incrementally injected under continuous stirring. Molar ratios of metal/reducing agent were varied as: 1:1, 1:2 and 1:4. Four reducing agents (all 0.1 M), (ascorbic acid, sodium citrate, sodium borohydride and DMAB) were used and the effect of

each was studied. For the formation of Ag colloidal suspsension, the mixtures were stirred for 3 h at either 60 ◦ C or room temperature when ascorbic acid and sodium citrate were used as reducants and at room temperature when sodium borohydride and DMAB were used as reductants. For the formation of nanoparticles, the mixtures were stirred for 3 h at 85 ◦ C for hydrolysis of dispersing agent (GA). A postsynthetic wash consisted in adding 25 mL of anhydrous ethanol to the obtained Ag powders. The mixture was centrifuged at 4500 rpm for 30 min. The supernatant was removed, and 5 mL of double-distilled water was added for futher cleansing. The mixture was centrifuged at 4500 rpm for another 30 min to remove supernants. The products were cleansed in total three times to obtain Ag powders with water and ethanol, alternately. 2.2. Microstructure characterization and composition analysis Absorption spectra of colloidal Ag were obtained with a PerkinElmer Lambda 35 ultraviolet–visible spectrophotometer (UV–vis, PerkinElmer, Fremont, CA), using 0.5 mL of colloidal suspension diluted to 5 mL. The morphology, particle size distribution, crystallinity and composition of the Ag-NPs was determined using a Tecnai F20 G2 transmission electron microscope (TEM) (FEI Company, Hillsboro, OH) equipped with X-ray energy dispersive spectrometer (EDS) and post-column Gatan Image Filter. Magnifications were calibrated using commercial cross-line grating replica and SiC lattice images [46]. The Ag-NPs were dispersed in absolute ethanol and deposited onto the carbon-coated copper grid. An Axis Ultra XPS (Kratos Analytical Incorporated, Chestnut Ridge, NY) was employed to identify the elemental composition in the Ag-NPs obtained at high vacuum (10−8 to 10−9 Torr) and anode mode was aluminum (K␣ : 1486.6 eV) monochromatic energy source. The resolution of element analysis was pass energy of 40 eV for individual and 160 eV for survey, respectively. A BIC ZetaPALS (Brookhaven Instruments Corporation, Holtsville, NY) was employed to measure the particle size distribution and the zetapotential () to evaluate the stability and electrokinetics of Ag-NPs. Zetapotential measurements were reported as the average  and standard deviation of four separate experimental runs with 15 measurements per sample to produce high signal-to-noise ratio. 2.3. Formation mechanism of nanoparticles The mechanism of Ag-NPs formation is summarized in Scheme 1. (1) The Ag+ cation is reduced to metallic Ag0 spontaneously (G = −75.45 kJ/mol); (2) Ag atoms form clusters, initially through hydrophobic-to-hydrophilic driven interactions and then through metallic bonding to form the core or cluster. The molecular orbitals in the cluster accommodate electrons, which are delocalized and occupy lowest energy orbitals to stablize the Ag cluster; (3) due to thermal diffusion, the Ag clusters aggregate to form polycrystals which also exhibit large surface tension; (4) surfactant (GA) is used to prevent Ag cluster from aggregation (at the macroscopic level) due to the ion–dipole intermolecular forces. Half reactions

Scheme 1. The mechanism of Ag-NPs formation.

I. Medina-Ramirez et al. / Colloids and Surfaces B: Biointerfaces 73 (2009) 185–191

187

Scheme 2. Oxidation reaction of ascorbic acid.

and their standard reduction potentials [47,48] are listed in Eqs. (i) and (ii): Ag+ (aq) + 1e− → Ag(s)

E ◦ = + 0.80 V

C6 H6 O6 (aq) + 2H+ (aq) + 2e− → C6 H8 O6 (aq)

(i) E ◦ = + 0.06 V (ii)

According to the half reaction standard reduction potential, the overall reaction was determined to have a potential of 0.793 V. Thermodynamically, therefore the redox reaction between Ag+ and C6 H8 O6 (2Ag+ (aq) + C6 H8 O6 (aq) = 2Ag(s) + 2H+ (aq) + C6 H6 O6 (aq)) occurs spontaneously (Scheme 2 displays the deprotonization reaction). Since H+ was released in the oxidation half reaction, the redox process was characterized by a significant decrease in the pH. This acidic condition was presumably the reason for the heterogeneous coagulation of the fine Ag polycrystalline nanoparticles due to ionic–dipole intermolecular forces. Although, a generalization, it can be assumed that the cluster phase is composed of a fluid-phase favoring dissolution and crystalline phase favoring aggregation, which are in equilibrium. The equilibrium between long-range repulsive forces and short-range attractive forces (e.g., van der Waals) will govern cluster size [49,50]. In small clusters, the decrease in cluster energy due to bonding of an additional particle on the surface of the cluster compensates for any additional repulsive force interaction, which dominates as the cluster size increases. Thus it can be seen that the packing fraction may be governed by phase separation kinetics, starting with a nucleation region, generating cluster phases [51].

Fig. 1. UV–vis spectra of the Ag-NPs, (a): using various reductants; and (b) various molar ratio of Ag and reductant.

3.2. XRD analysis of polymer-stabilized Ag-NPs Fig. 2 displays the X-ray diffraction (XRD) spectra of Ag-NPs powders obtained from the green synthesis procedure followed

3. Results and discussion 3.1. UV–vis analysis of polymer-stabilized Ag-NPs UV–vis spectra of the Ag-NPs are shown in Fig. 1a and b, from which it can be seen that metallic Ag-NPs were formed. The absorbance band detected at ca. 395–420 nm is the characteristic silver surface plasmon [49]. From the UV–vis spectra, the dispersity of colloidal solution was evaluated by comparison of the full width at half maximum (FWHM). The FWHM of all colloidal Ag specimens are essentially identical (85–100 nm), which indicates that the samples were monodispersed. The monodispersity is mainly due to the use of dispersing agent (GA). It is also seen that as particle sizes decrease (increase in absolute Zetapotential, see Section 3.5), a blue wavelength shift (from 418 to 409 nm) and peak broadening were observed. The molar ratio between AgNO3 and reductants displays influence on the Ag-NPs formation, which was seen through the difference of FWHM. However, the major reason to use excess reductants is to ensure that all Ag+ cations were reduced to metallic Ag.

Fig. 2. XRD patterns of the Ag-NPs.

188

I. Medina-Ramirez et al. / Colloids and Surfaces B: Biointerfaces 73 (2009) 185–191

Fig. 3. (a) TEM image of Ag-NPs with an insert of high-resolution image exhibting crystalline lattice fringes; (b) ring pattern of Ag-NPs; (c) single crystal electron diffraction; (d) EDS elemental composition analysis of Ag-NPs.

by drying for 10 h either in air at room temperature or at 110 ◦ C. The XRD patterns generally aligned well with that of an Ag faced-centered cubic (fcc) structure (powder diffraction file (PDF) 00-004-0783), which the frequency of merit is 0.32. In this case, the patterns did not to show any micro-distortion from cubic to other phase structure, which indicated that nicely crystallized structure was formed [52,53]. Notably, the intensity of the (1 1 1) peak at 2 of 38◦ (in Fig. 2) is the strongest and no peak splitting is detected. For

Fig. 4. XPS elemental composition analysis of Ag-NPs.

different Ag polycrystals obtained using various reducing agents, XRD spectra are completely overlapped and consistent with the standard one. This analysis indicated that the green synthesis is highly reproducible in the manufacturing of these nanomaterials.

3.3. TEM images of polymer-stabilized Ag-NPs The TEM micrograph (Fig. 3a) indicated that the average particle size of Ag-NPs was 40.2 nm. The particles were monodispersed with limited degree of aggregation. A large aperture to include large number of particles was used to obtain of the electron diffraction (ED) pattern (Fig. 3b). The ring patterns confirmed that highly crystalline Ag-NPs structure were formed. The d-spacings from ring patterns corresponded well to the d-spacings from XRD peaks with the fcc structure in Fig. 2. Single crystal pattern was obtained when only a single crytallline units were selected to diffract (Fig. 3c), which confirmed the formation of highly crystalline Ag-NPs. It was also noticed that Ag-NPs were subject to thermal damage under electron beam due to their ultrafine particle effect, which leads to a decrease in melting point.

Scheme 3. Particle orbital diagram of Silver.

I. Medina-Ramirez et al. / Colloids and Surfaces B: Biointerfaces 73 (2009) 185–191

Fig. 5. (a) Particle size distribution of Ag-NPs. (b) Zetapotential evaluation of 5 different silver nanoparticles (Ag-NPs, 1,2,3,4,5) in triplicate (3 columns/sample).

189

190

I. Medina-Ramirez et al. / Colloids and Surfaces B: Biointerfaces 73 (2009) 185–191

Therefore, TEM observation of Ag-NPs was carried out under weak illumination to avoid beam damage [54]. The X-ray energy dispersive spectroscopic (EDS) results were indicative of metallic silver formation. It can be concluded that the elements in Ag-NPs were mainly composed of Ag (Fig. 3d). The spectrum of the nanocomposite depicted that L␣1 of the Ag metal occurred at 2.984 eV and Ln at 2.806 eV. A satellite peak at L␤1 = 3.149 eV, arose from the multi-electron proces in the Ag atom, such as electron repulsion. Another peak at lower energy position 2.650 eV was also detected to be L␫. Peaks of K␣ = 22.162 eV and K␤ = 25.002 eV were observed due to the innermost electron emission [55]. The C and O atoms were observed, resulting from the surfactant (GA), as well as support carbon film on the copper grid. The Cu originated from the Cu grid. 3.4. XPS Analysis of polymer-stabilized Ag-NPs The XPS elemental analysis of the Ag-NPs is depicted in Fig. 4. The full (insert in Fig. 4) spectrum indicated the presence of the silver (Ag), oxygen (O) and carbon (C), which corresponded with the EDS analysis. Although an oversimplification, it can be concluded that metallic Ag is indicated by an asymmetric line shape with the peak tailing to the higher binding energy (BE). Fig. 4 (main) shows that the binding energies for Ag electron configurations of 3d5/2 and 3d3/2 of 366.1 eV and 372.1 eV, respectively with a difference of 6.0 eV. This is comparable to the standard Ag 3d binding energies (3d5/2 = 368.3 eV, 3d3/2 = 374.3 eV,  = 6.0 eV) [56]. It can be seen that the doublet separation of 3d (3d5/2 and 3d3/2 ) photoemission of Ag occured. The partial orbital diagram (including the outmost electrons) of naturally occuring Ag element is shown in Scheme 3 [57]. From the orbital diagram, it can be seen that 5s orbital is half-occupied and 4d sub-shell is completely full according to the Hund’s Rule. The lower energy (<20 eV) is required to remove the valence electrons. In our study, the most intense peak was due to the excitation of 3d electrons to the excited state which led to the binding energy changes. In addition, the subtle chemical shift of Ag peaks was found to be 2.2 eV for both 3d5/2 and 3d3/2 , respectively compared to standard metallic silver. The shift is caused by the chemical enviroment and fine particle size effect of Ag-NPs. 3.5. Particle size and zetapotential analysis of the polymer-stabilized Ag-NPs The Ag-NPs were precisely controlled in the regions between 10 and 50 nm and aggregation was successfully prevented using the dispersing agent (Fig. 5a). Two populations of the Ag-NP were found to be located in 13.5 nm (highest intensity set at 100% volume) and 15.7 nm (75.5% volume, relative to highest intensity volume) when ascorbic acid was used as reducing agent. Similarly, two populations were found in 10.3 nm (100% in volume) and 13.3 nm (30.6% in volume) when sodium citrate was used as reducing agent. Both green reducing agents produced essentially identical particles which exhibit similar size and stability. When NaBH4 was used, two populations of particles were located at 2.5 and 17.5 nm and the predominant size was about 10 nm; while the DMAB produced Ag-NPs with various size (mean bi-modal particle size was measured to be 2.5 and 85 nm). From the comparison of using various reductants, it can be concluded that the green reducing agents produced particles with slightly larger size than those from the toxic reducing agents. From our previous study [58], it was determined that ultrafine particles (<10 nm) may lead to mechanical instability. The electrokinetics analysis indicated that the fine particles exhibit large absolute zetapotential and particles were negatively charged (Fig. 5b). It was also noticed that the particles size from TEM is generally larger than the size from ZetaPALS. This is due to the

aggregation when the heat-treatment of nanoparticles is applied to obtain the TEM specimen. From our study, the time dependence of the zetapotential on the course of measurement was not observed, which indicated that the Ag-NPs colloidal suspensions were reasonably stable. The large zetapotential of the like (negative) charges minimized particle aggregation due to electrostatic repulsion [59,60]. This observation confirmed our proposed mechanism of Ag formation. 4. Conclusions The highly monodispersed nano-structured Ag particles were successfully developed and evaluated using advanced instrumentation techniques. It was concluded that the green synthesized materials were composed of near-spherical particles which were highly crystalline. The particle sizes were controlled in the range from 10 to 50 nm. The silver metal was found to be highly crystalline with fcc structure. The XPS study indicated that Ag binding energies for 3d5/2 and 3d2/3 were well-indexed to the standard metallic Ag. A subtle chemical shift was found due to its hydrophilic environment and small size effect. The magnitude of zetapotential was higher at low pH and its absolute value was significant to prevent the particle aggregation. Conflict of interest None of the authors has any direct (social) relationship with Elsevier publisher or the editorials staff of Colloids and Surfaces B: Biointerfaces including editors and reviewers. In addition, there is no direct relationship with the sponsors (FUMEC, AMC, TAMUK, and Robert A. Welch Foundation). Acknowledgements The authors are thankful to the Academia Mexicana de Ciencias (AMC), Fundación México Estados Unidos para la Ciencia (FUMEC), the Texas A&M University-Kingsville (TAMUK), College of Arts and Sciences Research and Development Fund (160310-00014), and the Robert A. Welch Foundation (departmental grant AC006) for their financial support. The technical support and facility access provided by the South Texas Environmental Institute and the Department of Chemistry (both at TAMUK), Microscopy and Imaging Center and Materials Characterization Facility (Dr. Gang Liang for his XPS analysis) at TAMU, College Station are duly acknowledged for allowing us to conduct the advanced instrumentation analyses. Dr. Thomas Hays is also aknowledged for his constructive discussion regarding this manuscript. References [1] C.J. Johnson, N. Zhukovsky, A. Cass, J.M. Nagy, Proteomics 8 (2008) 715. [2] L. Qiu, J. Franc, A. Rewari, D. Blanc, K. Saravanamuttu, J. Mater. Chem. 19 (2009) 373. [3] J.M. Campelo, D. Luna, R. Luque, J.M. Marinas, A.A. Romero, ChemSusChem 2 (2009) 18. [4] S. Rondinini, G. Aricci, Z. Krpetic, C. Locatelli, A. Minguzzi, F. Porta, A. Vertova, Fuel cells (2008) (Early View, No. 0), 1. [5] M.L. Gulrajani, D. Gupta, S. Periyasamy, S.G. Muthu, J. Appl. Polym. Sci. 108 (2008) 614. [6] S. Loher, O.D. Schneider, T. Maienfisch, S. Bokorny, W.J. Stark, Small 4 (2008) 824. [7] X. Huang, D. Du, X. Gong, J. Cai, H. Tu, X. Xu, A. Zhang, Electroanalysis 20 (2008) 402. [8] S.Y. Lee, H.J. Kim, R. Patel, S. Joon Im, J.H. Kim, B.R. Min, Polym. Adv. Technol. 18 (2007) 562. [9] Z. Yang, J. Lia, X. Yang, Xi. Xie, Y. Wu, J. Mol. Catal. A: Chem. 241 (2005) 15. [10] J.R. Davis, Metals Handbook, desk ed., 2nd ed., ASM International, Handbook Committee, San Jose, CA, 1998, pp. 658. [11] T.C. Chuang, Y.C. Liu, C.C. Wang, J. Raman Spectrosc. 36 (2005) 704. [12] L. Ming, Med. Res. Rev. 23 (2003) 697.

I. Medina-Ramirez et al. / Colloids and Surfaces B: Biointerfaces 73 (2009) 185–191 [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36]

D. Dorjnamjin, M. Ariunaa, Y. Key Shim, Int. J. Mol. Sci. 9 (2008) 807. J. Jiang, B. Winther-Jensen, E.M. Kjær, Macromol. Symp. 239 (2006) 84. P. Jain, T. Pradeep, Biotechnol. Bioeng. 90 (2005) 59. K. Kim, D. Han, J. Lee, H. Kim, Scripta Mater. 54 (2005) 143. M. Rai, A. Yadava, A. Gadea, Biotechnol. Adv. 27 (2009) 76. Q.L. Feng, J. Wu, G.Q. Chen, F.Z. Cui, T.N. Kim, J.O. Kim, J. Biomed. Mater. Res. Part A 52 (2000) 662. R. Tankhiwale, S.K. Bajpai, Colloids Surf. B 69 (2009) 164. N.L. Lala, R. Ramaseshan, L. Bojun, S. Sundarrajan, R.S. Barhate, Y. Liu, S. Ramakrishna, Biotechnol. Bioeng. 97 (2007) 1357. J.L. Elechiguerra, J.L. Burt, J.R. Morones, A. Camacho-Bragado, X. Gao, H.H. Lara, M.J. Yacaman, J. Nanobiotechnol. 3 (2005) 6. J.V. Rogers, C.V. Parkinson, Y.W. Choi, J.L. Speshock, S.M. Hussain, Nanoscale Res. Lett. 3 (2008) 129. A.M. Schrand, L.K. Braydich-Stolle, J.J. Schlager, L. Dai, S.M. Hussain, Nanotechnology 19 (2008) 13. K. Ishizu, H. Kakinuma, K. Ochi, S. Uchida, M. Hayashi, Polym. Adv. Technol. 16 (2005) 834. A.V. Ghule, K. Ghule, S. Tzing, Y. Ling, Eur. J. Inorg. Chem. 21 (2007) 3342. S. Ghader, M. Manteghian, M. Kokabi, R.S. Mamoory, Chem. Eng. Technol. 30 (2007) 1129. D. Radziuk, A. Skirtach, G. Sukhorukov, D. Shchukin, H. Möhwald, Macromol. Rapid Commun. 28 (2007) 848. H. Ma, B. Yin, S. Wang, Y. Jiao, W. Pan, S. Huang, S. Chen, F. Meng, ChemPhysChem 5 (2004) 68. S.S. Shankar, A. Ahmad, M. Sastry, Biotechnol. Prog. 19 (2003) 1627. L. Peponi, A. Tercjak, J. Gutierrez, H. Stadler, L. Torre, J.M. Kenny, I. Mondragon, Macromol. Mater. Eng. 293 (2008) 568. J.M. Domínguez-Vera, N. Gálvez, P. Sánchez, A.J. Mota, S. Trasobares, J.C. Hernández, J.J. Calvino, Eur. J. Inorg. Chem. 30 (2007) 4823. J. Compton, D. Kranbuehl, G. Martin, E. Espuche, L. David, Macromol. Symp. 247 (2007) 182. L.E. Macaskie, N.J. Creamer, A.M.M. Essa, N.L. Brown, Biotechnol. Bioeng. 96 (2007) 631. V. Bastys, I. Pastoriza-Santos, B. Rodríguez-González, R. Vaisnoras, L.M. LizMarzán, Adv. Funct. Mater. 16 (2006) 766. J. Zhang, Z. Liu, B. Han, D. Liu, J. Chen, J. He, T. Jiang, Chem. Eur. J. 10 (2004) 3531. J.A. Dahl, B.L.S. Maddux, J.E. Hutchison, Toward greener nanosynthesis, Chem. Rev. 107 (2007) 2228.

191

[37] D.D. Evanoff Jr., G. Chumanov, ChemPhysChem 6 (2005) 1221. [38] Y.S. Kang, S.W. Kang, H. Kim, J.H. Kim, J. Won, C.K. Kim, K. Char, Adv. Mater. 19 (2007) 475. [39] H. Ma, Y. Jiao, B. Yin, S. Wang, S. Zhao, S. Huang, W. Pan, S. Chen, F. Meng, ChemPhysChem 5 (2004) 713. [40] H. Jiang, S. Manolache, A.C. Wong, F.S. Denes, J. Appl. Polym. Sci. 93 (2004) 1411. [41] J.Q. Hu, Q. Chen, Z.X. Xie, G.B. Han, R.H. Wang, B. Ren, Y. Zhang, Z.L. Yang, Z.Q. Tian, Adv. Funct. Mater. 14 (2004) 183. [42] R.R. Naik, S.E. Jones, C.J. Murray, J.C. McAuliffe, R.A. Vaia, M.O. Stone, Adv. Funct. Mater. 14 (2004) 25. [43] V.R. Reddy, A. Currao, G. Calzaferri, J. Phys.: Conf. Ser. 61 (2007) 960. [44] D.D. Dionysiou, J. Environ. Eng. 130 (2008) 723. [45] S.M. Hussain, L.K. Braydich-Stolle, A.M. Schrand, R.C. Murdock, K.O. Yu, D.M. Mattie, J.J. Schlager, M. Terrones, Adv. Mater. 21 (2009) 11. [46] Z.P. Luo, Acta. Mater. 54 (2006) 47. [47] N. Garti, M.E. Leser, Polym. Adv. Technol. 12 (2001) 123. [48] P.C. Hiemenz, R. Rajagopalan, Principles of Colloid and Surface Chemistry, 3rd ed., Marcel Dekker Inc., New York, 1997, pp. 1–25. [49] D.V. Goia, J. Mater. Chem. 14 (2003) 451. [50] R. Kelsall, I.W. Hamley, M. Geoghegan, Nanoscale Science and Technology, 1st ed., Wiley Interscience, Malden, MA, 2005, pp. 282–330. [51] Z.L. Wang, Z.C. Kang, Functional and Smart Materials: Structural Evolution and Structure Analysis, Plenum, New York, 1998, pp. 405–466. [52] N.L. Lala, R. Ramaseshan, L. Bojun, S. Sundarrajan, R.S. Barhate, Y. liu, S. Ramakrishna, Biotechnol. Bioeng. 97 (2007) 1537. [53] T. Garino, M. Rodriguez, J. Am. Ceram. Soc. 83 (2000) 2709. [54] Z.L. Wang, J. Phys. Chem. B 104 (2000) 1153. [55] M.G. Guzmán, J. Dille, S. Godet, Proc. World Acad. Sci. Eng. Technol. 33 (2008) 2070. [56] C.D. Wagner, G.E. Muilenberg (Eds.), Handbook of X-ray Photoelectron Spectroscopy: A Reference Book of Standard Data for Use in X-ray Photoelectron Spectroscopy, 1st ed., Physical Electronics Division, Perkin-Elmer Corp., Eden Prairie, MN, 1979, p. pp. 121. [57] G. Umarji, S. Ketkar, R. Hawaldar, S. Gosavi, K. Patil, U. Mulik, D. Amalnerkar, Microelectron Int. 25 (2008) 46. [58] J. Liu, A.C. Co, S. Paulson, V.I. Birss, Solid State Ionics 177 (2006) 377. [59] G.M. Dougherty, K.A. Rose, J.B. Tok, S.S. Pannu, F.Y. Chuang, M.Y. Sha, G. Chakarova, S.G. Penn, Electrophoresis 29 (2008) 1131. [60] Y. Sun, Y. Liu, G. Zhao, X. Zhou, J. Gao, Q. Zhang, J. Mater. Sci. 43 (2008) 4625.