poly(N-vinyl-2-pyrrolydone) Biodegradable triblock copolymer as stabilizer and reductant

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Macromolecular Nanotechnology

Gold-copolymer nanoparticles: Poly(e-caprolactone)/poly(N-vinyl-2pyrrolydone) Biodegradable triblock copolymer as stabilizer and reductant Angel Leiva a,*, César Saldías a, Caterina Quezada a, Alejandro Toro-Labbé a, Francisco J. Espinoza-Beltrán b, Marcela Urzúa c, Ligia Gargallo a, Deodato Radic a a

Departamento de Química Física, Facultad de Química, Pontificia Universidad Católica de Chile, Casilla 302, Correo 22. Santiago, Chile Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional (CINVESTAV-QUERETARO), Apartado Postal 1-798, Arteaga # 5, Col. Centro, Santiago de Querétaro, Qro. 76001, México c Departamento de Química, Facultad de Ciencias, Universidad de Chile, Casilla 653, Correo Central, Santiago, Chile

a r t i c l e

i n f o

Article history: Received 9 March 2009 Received in revised form 7 July 2009 Accepted 10 August 2009 Available online 14 August 2009 Keywords: Gold-copolymer Nanoparticles Biodegradable Copolymer

a b s t r a c t Block copolymers have been extensively used in the synthesis of many types of nanoparticles, where generally are considered as stabilizer and protective agent. In this work a double function of the biodegradable triblock copolymer poly(N-vinyl-2-pyrrolidone)-bpoly(e-caprolactone)-b-poly(N-vinyl-2-pyrrolidone), (PVP–PCL–PVP) in the gold nanoparticle-copolymer synthesis is reported. Gold-copolymer composed nanoparticles were synthesized using the triblock copolymer (PVP–PCL–PVP) and potassium tetrachloro aurate (III), both in aqueous solution. The copolymer work as both, reductant and stabilizer agent. The obtained nanoparticles were characterized by FT-IR, dynamic light scattering (DLS), atomic force microscopy (AFM) and transmission electron microscopy (TEM). The shape and the size of the obtained nanoparticles are dependent on the copolymer/salt of gold concentration ratio used in the synthesis. To complement the experimental results about the copolymer role in the nanoparticles synthesis, computational tools were used to characterize the reactivity of the reactant species. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction The nanoscience revolution that happened in the 1990s has had a great impact in the definition of the current and futures interests of scientist of different disciplines. Actually, we are witness of important transformations in the field of the chemical nanoscience, by one hand the synthesis for control the size and the shape of organic and inorganic nanostructures is developed, and for other the activities are focused toward connecting the design of nanoparticles with their structure and function [1]. Polymers with some defined characteristics such as, size, shape, architecture and chemical functionality, either from natural or synthetic origin, are between the most promising and * Corresponding author. Tel.: +56 2 6864041; fax: +56 2 6864744. E-mail address: [email protected] (A. Leiva). 0014-3057/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2009.08.005

versatile building blocks for nanostructured materials. Whereas the characteristic length and shape of the nanostructures generated mainly by self-assembly and self-organization, are controlled by the size and geometry of the starting polymeric units, different functions can be achieved controlling the chemical composition and functional groups included in the polymers. Thus, properties such as controlled adhesion and biocompatibility, the design of intelligent and responsive nanostructures, and others, can be achieved with a controlled design process [2]. Methods that allow the preparation of polymeric nanoparticles with varied and complex structures and functions are actually matter of great interest based in the notion that such nanoparticles may offer enhanced physical, chemical, or biological properties when are optimally designed for specific applications [3]. Nanoparticles have a characteristic high surface to volume ratio, providing

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b

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sites for the efficient adsorption of the reacting substrates leading to unusual size dependent chemical reactivity. Polymers are also broadly used in the nanostructures development made up with metals [4]. Since the pioneer report of Helcher in 1718, indicating that starch stabilizes water-soluble gold particles, it has been known that polymers, favor the isolation of gold nanoparticles [5,6]. With the considerably improved understanding of the parameters leading to the stabilization of gold nanoparticles and of their quantum size-related interest, there has been a revival of activity in the field of polymer-stabilized gold nanoparticles [4]. The most commonly used polymers for the stabilization of gold nanoparticles are poly(vinyl pyrrolydone) (PVP) and poly(ethylene glycol) [5]. Although there are a variety of ways to achieve metal-polymer nanoparticles [7,8], two different approaches dominate. The first one consists of the in situ synthesis of the nanoparticles in the polymer matrix either by reduction of the metal salts dissolved in that matrix [9] or by evaporation of the metals on the heated polymer surface [10]. The second one, less frequently used, involves polymerization of the matrix around the nanoparticles [11]. In this work, we report copolymer-gold nanoparticles synthesized by direct reaction between the triblock copolymer poly(N-vinyl-2-pyrrolidone)-b-poly(e-caprolactone)-b-poly(N-vinyl-2-pyrrolidone) (PVP–PCL–PVP) and KAuCl4 in water. In the synthesis the copolymer executes a double action as reducer and stabilizer. A similar behavior has been reported for PEO–PPO–PEO copolymer and PVP [12–13].

2. Experimental

2.3. Synthesis of DTPA–PCL–DTPA Four grams of PCL-diol and 2 g of DTPA were dissolved in 22 ml of THF and 1.6 g of DCC and 40 mg of DMAP dissolved in 10 mL of THF were added. The reaction mixture was stirred for 48 h at room temperature. The solution obtained was filtered and treated with DCM to precipitate traces of dicyclohexylurea (DCU). The solution was filtered, extracted three times with 50 ml of 5% aqueous NaCl, and the organic phase dried over anhydrous magnesium sulfate. DCM was evaporated under reduced pressure and the oily product dried under vacuum for 48 h. 2.4. Synthesis of HS–PCL–SH Two grams of DTPA–PCL–DTPA were dissolved in 10 mL of anhydrous DMF, 0.61 g of DTT was added and the reaction mixture was stirred at room temperature for 24 h. The product was precipitated in cold water, isolated by filtration, washed with water and dried under vacuum for 48 h. 2.5. Synthesis of PVP-b-PCL-b-PVP About 1.3 g of HS–PCL–SH, 3.6 ml de VP, 0.055 g of AIBN and 15 mL of anhydrous DMF were mixed and the solution purged with nitrogen for 10 min. The polymerization was performed at 80 °C under nitrogen with continuous stirring for 16 h. The polymer was isolated and purified by repeated precipitation in cold diethyl ether and dried under vacuum for 48 h. 2.6. Nanoparticles synthesis

2.1. Reagents 0

Poly(e-caprolactone)diol (M n 2000 g/mol) (PCL-diol), 3,3 dithiobis(propionic acid) (DTPA), dicyclohexyl carbodiimide (DCC), 4-(dimethylamino)pyridine (DMAP), 1,4 dithiothreitol (DTT), N-vinyl-2-pyrrolidone (VP), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAC), magnesium sulfate and Potassium tetrachloro aurate (III) were purchased from Aldrich and tetrahydrofuran (THF), dichloromethane (DCM), diethyl ether (DEE), and sodium chloride purchased from Merck. DMF was purified by distillation under reduced pressure and 2,2,-azobis(isobutyronitrile) (AIBN) was recrystallized from ethanol. All others chemical were used without prior treatment. Water purified by Millipore Milli-Q system (resistivity greater than 18.0 MXcm) was used in all the experiments.

2.2. Copolymer synthesis PVP–PCL–PVP block copolymer composed of a poly(ecaprolactone) central block and poly(N-vinyl 2-pyrrolidone) arms was synthesized in three consecutive steps by the reported method [14,15] for copolymers of N-(2hydroxypropyl)methacrylamide and N-vinyl-2-pyrrolidone with D,L-lactide and poly(e-caprolactone), with minor modifications indicated below.

The nanoparticles were synthesized by direct reaction of copolymer and KAuCl4 in water at different concentration ratios, according to the following example procedure. About 5.0 ml of PVP–PCL–PVP aqueous solution (1.12 g/L) and 2.5 mL of aqueous of KAuCl4 (0.392 g/L) were mixed and vigorously stirred during 2 h at 70 °C. Finally the reaction mixtures were centrifuged to separate the nanoparticles and these were washed with ultra pure water three times. 2.7. Nanoparticles characterization Nanoparticles were characterized by dynamic light scattering DLS, transmission electron microscopy TEM, atomic force microscopy AFM. DLS measurements were made using a Nicomp 370 particle size analyzer. TEM measurements were made using a FEI Tecnai G2 F20 S-Twin high resolution transmission electron microscope (HRTEM). AFM measurements were performed in an AFM, Dimension 3100 – NanoScope IV, Digital Instruments. 3. Results and discussion 3.1. Polymer characterization Polymers were characterized by Fourier transform infrared spectroscopy (FT-IR) in KBr and by 1H NMR in CDCl3.

1 H NMR spectra of PVP–PCL–PVP in CDCl3 include the characteristic signals of PVP, the –CH-backbone protons at 3.7 ppm, the –CH2-protons adjacent to the nitrogen atom at 3.2 ppm, the –CH2-protons in the backbone, the protons adjacent and b to the carbonyl group, at 2.0, 2.3 and 1.7 ppm respectively. For the PCL blocks, the –CH2O- and –CH2–CO–O-protons appear at 4.0 and 2.3 ppm, respectively, and the central –CH2- at 1.4–1.7 ppm. The signals at 2.9 and 2.95 ppm correspond to the –CH2- protons between the carbonyl group and the sulfur atoms that attach the three PVP arms to the PCL core. The FT-IR spectra, also corroborate the proposed copolymer structure, with characteristic signals due to the VP units (amide carbonyl group at 1657 cm-1) and ecaprolactone units (carbonyl group and methylene CH at 1734 and 2951 cm1, respectively), and hydroxyl terminal groups. The chemical structure of the copolymer is shown in Scheme 1. The number average molecular weight, (M n ), of the copolymer was determined by size-exclusion chromatography employing three PLgel 5 lm mixed – c Polymer Labs columns and an Optilab DSP Wyatt technology Corporation IR detector with chloroform as eluent; the universal calibration curve of polystyrene in the same solvent was used. M n of 6400 g mol1and polydispersity index of 1.6 were obtained. Since VP was polymerized onto HO–PCL–OH central block of 2000 g mol1, the increment in the copolymer molecular weight is due to the increase in the VP content of the copolymer. Therefore, the VP content of 68.75% was determined by simple difference. The relative PCL and PVP contents of the copolymers were also determined by 1H NMR; and the molecular weight of the copolymer was estimated from its composition and the known molecular weight of the PCL central block, good agreement of these values with the SEC results was observed.

3.2. Nanoparticles synthesis and characterization During the synthesis reaction of gold-copolymer nanoparticles, the initial yellowish colored solution of block copolymer with AuCl4, change rapidly to a characteristic purple-red color of the gold nanoparticles. Fig. 1 shows the UV–vis absorption spectra of samples obtained during formation of nanoparticles at different time. The absorption band observed at approx. 575 nm is assigned to the surface plasmon resonance of small gold nanoparticles. The absorption band intensity increases with the reaction time until a constant value at 60 min of reaction. Generally, in the metallic nanoparticles synthesis, reductants such as sodium borohydride or sodium citrate

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besides stabilizers are used. In our case, nanoparticles synthesis was carried out without classical reductant agent, and the copolymer act as both reductant and stabilizer. Considering the previously mentioned and the fact that the used copolymer is biodegradable, the method of synthesis of nanoparticles reported in this work is environmentally friendly; this have special interest considering the actual environmental concern. In most of the works that approach the nanoparticles synthesis in presence of PVP, this polymer is considered as stabilizer or protective agent [16]. PVP has been used in the silver and gold nanoparticles synthesis as stabilizer and capping material in water and in ethylene glycol/water mixtures [17,18]. There are some works in which it is reported that PVP acts as stabilizer and/or reducer in the synthesis of metallic nanoparticles, however it is not clear how PVP reduces Au III. Xia and col. [17] report the reduction of noble metals in aqueous solution by the hydroxyl end groups of PVP, with the formation of carboxyl acid ends. On the other hand Hoppe and col. [16] propose that PVP acts as the reducing agent suffering a partial degradation during the nanoparticles synthesis. The use of alcohols as reducers has been known by decades as a convenient route for metallic colloids synthesis [19–23], it is also known that their reducer power decrease with the increment of the alkyl chain length, for this reason polymers with terminal hydroxyl groups would be very useful reductants in the controlled synthesis of metallic nanostructures. The experimental results obtained so far allow us to postulate that are the hydroxyl tail groups those that reduce Au III in the nanoparticles synthesis. Fig. 2 shows the FT-IR spectra of copolymer and copolymer-gold nanoparticle. The main features are, the intensity decrease of the signal at 3400 cm1 assigned to the hydroxyl ends groups and the apparition of a small signal that could correspond to the aldehyde group stretching H-(C@O) at  2800 cm1. Both observations, intensity decrease of hydroxyl groups signal and the apparition of an aldehyde group signal, allow us to suggest that the reduction of gold take place by copolymer hydroxyl end groups oxidation to aldehyde. The aldehyde formation is suggested considering the FT-IR results and the fact that the oxidation of primary alcohols to acids involves more rigorous experimental conditions than the oxidation to aldehydes, which possess a lower oxidation state than acids [24,25]. By dynamic light scattering the size distribution of the nanoparticles and the respectives diffusion coefficients were determined. The obtained results are summarized

Scheme 1.

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Absorbance / a.u.

2

Time

400

600

800

1000

1200

Wavelenght / nm Fig. 1. UV–vis spectra of nanoparticles synthesis at different time of reaction.

Au-Copolymer Nanoparticles Copolymer

1.0

Transmitance

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0

0.8

0.6

3500

3000

2500

2000

1500

1000

500

0

-1

Wavenumber (cm ) Fig. 2. FT-IR spectra for copolymer and gold-copolymer nanoparticles.

in Table 1. The main feature is that nanoparticles size decreases and consequently the diffusion coefficients in-

creases when copolymer/gold mass ratio feed increases. In addition, wide size distributions are obtained, what

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w/w

(–OH) groups/KAuCl4 mol/mol

Copolymer/KAuCl4 feed ratio 10/1 1.2 30/1 3.5 50/1 6.0

Nanoparticles average diameter (nm)

Mean diffusion coefficient (cm2/s)  108

197.7 ± 39.6 128.8 ± 41.9 105.8 ± 36.2

2.134 3.276 3.989

indicates a high dispersion of nanoparticles size, just as Fig. 3 illustrates for nanoparticles obtained with 50/1 w/ w copolymer/gold salt feed ratio. Fig. 4 shows TEM micrographs of obtained gold-copolymer nanoparticles at different copolymer/gold feed ratio. Well defined geometric shapes were observed. Hexagonal and triangular-shaped nanoplates of different sizes covering a wide interval of sizes from 50 to 300 nm approximately were observed. This type of nanostructures, have a wide scientific interest, due to the dramatic effect that the shape anisotropy has in electronic, optical and catalytic properties of noble metal structures. The synthesis of noble metal nanostructures as nanoplates, nanorods and nanocubes is actually an area of great research activity and different shapes of nanoparticles have been reported [26]. For gold nanoplates, various chemicals such as PEO–PPO– PEO block copolymers [27], 2-phenylenediamine [28], dimyristoyl-L-a-phosphatidyl-DL-glycerol [29] and salicylic acid [30] have been used as capping agents to control the growing rate of different crystal facets of gold leading to gold nanostructures with plate-like shapes. Recent promising works in this field are the preparation of gold nanoplates by chemical reduction of hydrogen tetrachloroaurate by a reduced amount of sodium citrate in the presence of

poly(N-vinyl-2-pyrrolidone) at a boiling temperature, which kinetically adjusts the reaction pathway to a stepwise selfseeding growth of the nanoplates [31]. The surface morphology of the nanoparticles deposited over mica by spin coating was analyzed by atomic force microscopy. Tapping mode was used for this purpose. Fig. 5 a shows the AFM image of nanoparticles sample spin coated over mica. This result reveals nanometric particles on the flat surface. These particles have a planar shape like a plate, which can be clearly observed from a cross section image in Fig. 5b. To support the experimental results about the copolymer reactivity in the nanoparticles synthesis, the dual descriptor of reactivity and selectivity Df(r) index was used [32,33]. The dual descriptor helps identify simultaneously nucleophilic and electrophilic regions within a molecule. It is defined as the difference between the nucleophilic and electrophilic Fukui functions [34,35], these latter expressed in terms of the frontier molecular orbital densities:

Df ðrÞ  ½f þ ðrÞ  f  ðrÞ  ½ql ðrÞ  qh ðrÞ

1.0

Intensity (a.u.)

0.8

0.6

0.4

0.2

0.0 0

200

400

size (nm) Fig. 3. Size distribution obtained by DLS for gold-copolymer nanoparticles (feed ratio 50/1 w/w).

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Table 1 Nanoparticles size at different feed ratios obtained by dynamic light scattering.

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Fig. 4. TEM image of gold-copolymer nanoparticles, at different feed ratios. (a) 10/1, (b) 30/1, (c) 50/1, w/w copolymer/KAuCl4.

where r is the space coordinate; f+(r) and f-(r) are the electrophilic and nucleophilic Fukui functions respectively; ql(r) and qh(r) are the densities associated to the lowest unoccupied and highest occupied molecular orbitals at point r. Using the above definition, molecular sites with Df(r) > 0 are favored for a nucleophilic attack whereas if sites presenting Df(r) < 0 are favored for an electrophilic attack [32,33]. The geometries of copolymer model have been fully optimized using the B3LYP methodology [36,37] with the standard 6-311G basis set, as implemented in the Gaussian 03 package [38]. This level of theory gives a good description of the electron density and orbital energies of the copolymer studied here [39]. The resulting map of the dual descriptor is displayed in Fig. 6, it is observed that most end hydroxyl groups in the copolymer act as nucleophiles (Df(r) < 0) while others do not present a marked nucleophilic or electrophilic character, this is probably due to steric effects and specific local interactions with neighboring groups. On the other hand, the map of the dual descriptor for AuCl4 ion, obtained using the methodology B3PW91 [40], using pseudopotentials and LANL2DZ basis set, displayed in Fig. 6

shows that the Au atom presents a clear electrophilic character. These results indicate that in front of the AuCl4 complex the copolymer can initially act as nucleophile to reduce the gold atom. The sites in those that Df(r) is > 0, would stabilize the gold nanoparticles by interaction with the metal surface. These results are consistent with the reducing power of the copolymer ends hydroxyl groups and with the experimental results previously discussed. In conclusion, in this work a new method of gold nanoparticles synthesis is reported. The method has the advantage of being environmentally friendly, since a biodegradable copolymer is used as stabilizer and reducer. The double participation of the block copolymer as stabilizer and reducer is approached by means of classic techniques and theoretical chemistry, obtaining a good agreement of results. Experiments focused to optimize the synthesis process of gold-copolymer nanoparticles and theoretical studies directed to clarify the reactivity between copolymer and gold salt, and the interactions involved in the stabilization of the metallic nanoparticles are actually in development.

Fig. 5. (a) AFM image of nanoparticles spin coated over mica, (b) profile of height of the corresponding cross-section.

Fig. 6. Df(r) map of copolymer model and AuCl4.

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Acknowledgments The authors thank Fondecyt Grants 1080238, 1090460 and 7070187, FONDAP 11980002 (CIMAT), for partial financial support of this research. C.S. thanks to Conicyt for Doctoral fellowship.

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References [1] Stupp SI. Introduction: functional nanostructures. Chem Rev 2005;105(4):1023–4. [2] Antonietti M. Nanostructured materials: self-organization of functional polymers. Nature Mater 2003;2:9–10. [3] Wang X, Hall JE, Warren S, Krom J, Magistrelli JM, Rackaitis M, et al. Synthesis, characterization, and application of novel polymeric nanoparticles. Macromolecules 2007;40:499–508. [4] Daniel MC, Astruc D. Gold nanoparticles: assembly, supramolecular chemistry, quantum-size-related properties, and applications toward biology, catalysis, and nanotechnology. Chem Rev 2004; 104:293–346. [5] Hayat MA, editor. Colloidal gold: principles, methods, and applications. San Diego: Academic Press; 1989. [6] Napper DH. Polymeric stabilization of colloidal dispersions. London: Academic Press; 1983. [7] Ziolo RF, Giannelis EP, Weinstein BA, O’Horo MP, Ganguly BN, Mehrotra V, et al. Matrix-mediated synthesis of nanocrystallineFe2O3: a new optically transparent magnetic material. Science 1992;257:219–23. [8] Jordan R, West N, Chou YM, Nuyken O. Nanocomposites by surface-initiated living cationic polymerization of 2-oxazolines on functionalized gold nanoparticles. Macromolecules 2001;34: 1606–11. [9] Selvan ST, Spatz JP, Klock HA, Moller M. Gold-polypyrole core-shell particles in diblock copolymer micelles. Adv Mater 1998;10:132–4. [10] Sayo K, Deki S, Hayashi SA. Eur Phys J 1999;D9:429–32. [11] Corbierre MK, Cameron NS, Sutton M, Mochrie SGJ, Lurio LB, Rühm A, Lennox RB. Polymer-stabilized gold nanoparticles and their incorporation into polymer matrices. J Am Chem Soc 2001; 123(42):10411–2. [12] Sakai T, Alexandridis P. Mechanism of gold metal ion reduction, nanoparticle growth and size control in aqueous amphiphilic block copolymer solutions at ambient conditions. J Phys Chem B 2005;109(16):7766–77. [13] Lin JM, Lin TL, Jeng US, Zhong YJ, Yeh CT, Chen TY. Fractal aggregates of the Pt nanoparticles synthesized by the polyol process and poly(N-vinyl-2-pyrrolidone) reduction. J Appl Cryst 2007;40:s540–3. [14] Kang N, Leroux JC. Triblock and star-block copolymers of N-(2hydroxypropyl)methacrylamide or N-vinyl-2-pyrrolidone and d, l-lactide: synthesis and self-assembling properties in water. Polymer 2004;45(26):8967–80. [15] Lele BS, Leroux JC. Synthesis and micellar characterization of novel amphiphilic A–B–A triblock copolymers of N-(2hydroxypropyl)methacrylamide) or N-vinyl-2-pyrrolidone with poly(-caprolactone). Macromolecules 2002;35:6714–23. [16] Hoppe C, Lazzari M, Pardiñas-Blanco I, López Quintela MA. One-step synthesis of gold and silver hydrosols using poly(N-vinyl-2pyrrolidone) as a reducing agent. Langmuir 2006;22(16):7027–34. [17] Xiong Y, Washio I, Chen J, Cai H, Li ZY, Xia Y. Poly(vinyl pyrrolidone): a dual functional reductant and stabilizer for the facile synthesis of noble metal nanoplates in aqueous solutions. Langmuir 2006; 22(20):8563–70.

[18] Eustis S, Hsu H, El-Sayed MA. Gold nanoparticle formation from photochemical reduction of Au3+ by continuous excitation in colloidal solutions. A proposed molecular mechanism. J Phys Chem B 2005;109(11):4811–5. [19] Xiong Y, Chen J, Wiley B, Xia Y. Understanding the role of oxidative etching in the polyol synthesis of Pd nanoparticles with uniform shape and size. J. Am. Chem. Soc. 2005;127(20):7332–3. [20] McLellan J, Xiong Y, Hu M, Xia Y. Surface-enhanced Raman scattering of 4-mercaptopyridine on thin films of nanoscale Pd cubes, boxes, and cages. Chem Phys Lett 2006;417:230–4. [21] Gugliotti LA, Feldheim DL, Eaton BE. RNA-mediated metal–metal bond formation in the synthesis of hexagonal palladium nanoparticles. Science 2004;304:850–2. [22] Wiley B, Herricks T, Sun Y, Xia Y. Polyol synthesis of silver nanoparticles: use of chloride and oxygen to promote the formation of single-crystal, truncated cubes and tetrahedrons. Nano Lett 2004;4(9):1733–9. [23] Kim KS, Choi S, Cha JH, Yeon SH, Lee H. Facile one-pot synthesis of gold nanoparticles using alcohol ionic liquids. J Mater Chem 2006;16:1315–7. [24] Tojo G, Fernández M. Oxidation of primary alcohols to carboxylic acids, cap. 7. New York: Springer; 2007. p. 105–109. [25] Molander GA, Petrillo DE. Oxidation of hydroxyl-substituted organotrifluoroborates. J Am Chem Soc 2006;128(30):9634–5. [26] Pardiñas-Blanco I, Hoppe CE, Piñeiro-Redondo Y, López-Quintela MA, Rivas J. Formation of Gold branched plates in diluted solutions of poly(vinylpyrrolidone) and their use for the fabrication of nearinfrared-absorbing films and coatings. Langmuir 2008;24:983–90. [27] Sakai T, Alexandridis P. Size- and shape-controlled synthesis of colloidal gold through autoreduction of auric cation by poly(ethylene oxide)-poly(propylene oxide) block copolymers in aqueous solutions at ambient conditions. Nanotechnology 2005; 16(7):S344–53. [28] Sun X, Dong S, Wang E. Large-scale synthesis of micrometer-scale single-crystalline au plates of nanometer thickness by a wetchemical route. Angew. Chem. Int. Ed. 2004;43(46):6360–3. [29] Ibano D, Yokota Y, Tominaga T. Preparation of gold nanoparticles protected by an anionic phospholipid. Chem Lett 2003;32(7):574–5. [30] Malikova N, Pastoriza-Santos I, Schierhorn M, Kotov NA, Liz-Marzán LM. Layer-by-layer assembled mixed spherical and planar gold nanoparticles: control of interparticle interactions. Langmuir 2002;18(9):3694–7. [31] Ah CS, Yun YJ, Park HJ, Kim WJ, Ha DH, Yun WS. Size-controlled synthesis of machinable single crystalline gold nanoplates. Chem Mater 2005;17(22):5558–61. [32] Morell C, Grand A, Toro-Labbé A. Theoretical support for using the Df(r) descriptor. Chem Phys Lett 2006;425:342–6. [33] Morell C, Grand A, Toro-Labbé A. New dual descriptor for chemical reactivity. J Phys Chem A 2005;109:205–12. [34] Geerlings P, De Proft F, Langenaeker W. Chem Rev 2003;103(5):1793–874. [35] Parr RG, Yang W. Density-functional theory of atoms and molecules. New York: Oxford University Press; 1989. [36] Becke AD. J Chem Phys 1993;98:5648–52. [37] Lee C, Yang W, Parr RG. Phys Rev 1988;B37:785–9. [38] Frisch MJ, Trucks GW, Schlegel HB, et al. GAUSSIAN 03, Revision C. 02. Wallingford, CT: Gaussian, Inc.; 2004. [39] Bulat F, Chamorro E, Fuentealba P, Toro-Labbe A. J Phys Chem A 2004;108:342–9. [40] Redfern PC, Blaudeau JP, Curtiss LA. J Phys Chem A 1997;101(46):8701–5.